Optimal placement of corals in the saltwater aquarium: An expert report on factors, compatibility and design

Introduction: The art and science of coral placement in the reef aquarium

Successfully maintaining a reef aquarium is a fascinating synthesis of biological understanding and aesthetic sensitivity. A key aspect that determines the health, growth, and visual impact of corals is their correct placement within the aquarium ecosystem. This task is complex, as each coral species has specific environmental requirements.1 Light availability, water flow, required habitat and interaction with neighboring organisms are critical variables that must be carefully considered.1 Errors in placement can lead to stress, reduced growth, loss of color or even death of the animals.4

This report aims to provide advanced aquarists and professionals with a scientifically sound and practical foundation for optimal coral placement. It covers the key factors of light, current, and aggressiveness, analyzes the compatibility of different coral groups (SPS, LPS, soft corals), compares different tank designs, provides detailed recommendations for commonly kept species, explains stress-free acclimation methods, illuminates the long-term dynamics of coral growth, and concludes with aquascaping principles for a harmonious reef appearance. Understanding the specific needs of each individual coral forms the essential basis for all placement decisions.1

1. Key factors for successful coral placement

Choosing the right location for a coral in a reef aquarium depends on the interplay of several environmental factors. The three most important are light, water flow, and the aggressive potential of the coral and its neighbors.

1. Light: Intensity (PAR), spectrum and adjustment

Light is the primary energy source for most corals kept in aquariums. Embedded symbiotic algae, the zooxanthellae, perform photosynthesis and supply the coral with nutrients.3 Light significantly influences the growth, coloration and general vitality of corals.4

The Photosynthetically Active Radiation (PAR) is the established measure of the light intensity that can be used by zooxanthellae for photosynthesis.1 PAR requirements vary considerably between different coral groups and even within species and color variants.1 In general, small-polyp stony corals (SPS) require higher PAR values, often in the range of 200 to over 450 µmol/m²/s.9 Large-polyp stony corals (LPS) and many soft corals, on the other hand, often cope with lower intensities of 50 to 150 µmol/m²/s.1 Specific examples illustrate the range: Acropora species can require 200 to over 700 PAR 10, Montipora 100 to 400 PAR 10, Euphyllia species typically 100 to 250 PAR 14, Zoanthus 80 to 150 PAR 10 and Leptoseris prefers moderate 75 to 125 PAR.21 Since light intensity decreases with increasing water depth, measuring PAR values ​​at different locations in the aquarium using a PAR meter is highly recommended in order to be able to accurately zone the placement of different corals.9 Measurements often show surprisingly high values ​​in the upper range, which can already mean stress for many corals.9

In addition to the intensity, the light spectrum an important role. Different wavelengths, especially in the violet and blue range, are crucial for stimulating fluorescent pigments in the corals and thus influencing their color appearance.4 Red light can be problematic for some sensitive corals.8 The optimal spectrum depends on the well-maintained coral population – corals with fluorescent pigments (glow under blue light) benefit from different spectra than those with striking pigments (colored in daylight).8 Modern LED lighting often offers the possibility to adjust the spectrum.8

The strong dependence of placement on specific PAR values 1 has far-reaching consequences. Inadequate or incorrectly zoned lighting not only affects growth and color 4, but fundamentally limits the selection of corals that can be maintained and the overall potential for aquascaping. The knowledge and control of PAR values ​​through measurement 9 and dimming 8 is therefore a basic requirement for the successful care of various corals, especially in mixed tanks.23

Another critical aspect is that Light acclimatizationCorals must be slowly acclimated to the lighting conditions in the new aquarium to avoid light stress and the dreaded bleaching (loss of zooxanthellae).8 The physiological adaptation of zooxanthellae to new light intensities is a slow process that can take days to weeks.14 “Light shock” during transfer is a significant but avoidable stress factor.8 The common strategy is to initially place newly acquired corals in deeper, shadier areas of the aquarium and gradually move them to their final, brighter location over several days or weeks.8 For new lamps, the power should also be increased gradually.8 This need for gradual adaptation implies that the initial placement can often only be temporary, which must be taken into account in aquascaping planning – space is needed for “intermediate stations”.

1. Water current: meaning, types and coral-specific needs

Water flow is another vital factor in the reef aquarium.29 It fulfils multiple essential functions: It transports food particles to the sessile corals 3, removes waste products, sediments and excess mucus from the coral surface (“sliming”) 3, promotes gas exchange (oxygen supply, CO₂ removal) 32, prevents the accumulation of detritus in low-flow zones 30 and supports nutrient absorption.1 Moderate movement of the coral polyps can also contribute to a more even light supply.30

In the aquarium you can find various Flow types realize:

• Laminar flow: Rectified, carries the risk of “dead zones”.32
• Turbulent flow: Irregular, alternating directions, often preferred as it is closer to natural conditions and promotes better distribution.32
• Intermittent/Pulsating/Wavy Flow: Simulate the dynamic conditions of a natural reef (e.g. waves, tidal currents) and provide variety.14 An annular basic flow, generated by two opposing pumps, can be modified by additional pumps or interval circuits to create turbulence.30

The coral-specific flow needs vary greatly 1:

• SPS Corals: Often require a strong, often turbulent current, such as that found on the reef top.5 Examples are Acropora, Stylophora and Montipora.12
• LPS corals: Usually prefer a moderate to weak, often indirect current.1 Their large, fleshy polyps can easily be damaged by too strong or direct currents or remain retracted.14 Examples of LPS with lower flow rates are Caulastrea, Blastomussa, Plerogyra and Fungia.37 Euphyllia species require moderate, indirect, often pulsating flow.14
• Soft corals: Their needs vary greatly. Some species, such as Sarcophyton (mushroom leather coral) or gorgonians, appreciate strong currents. 29, while others, like many disc anemones (“current muffles”), prefer quieter zones.1

The current is therefore more than just a physical force; it functions as a complex transport system in the aquarium.3 Insufficient flow leads to “dead zones” 32in which waste materials accumulate 30 and nutrients may be lacking. A current that is too strong or directed directly at the coral can cause mechanical damage to the tissue.14 Suboptimal flow can therefore lead to a cascade of problems, from direct damage to nutrient deficiencies to the accumulation of local pollutants or sediment deposits, which can impair coral growth and health in the long term.21 The ideal current must be strong enough for transport, but gentle enough to avoid damage, and precisely adapted to the needs of the respective coral.35

This requires careful Placement and selection of flow pumps. Multiple pumps are usually necessary to ensure a uniform flow throughout the pool and to avoid dead zones.29 Important distances must be observed: sufficient distance from the corals to avoid direct bombardment (at least 15-50 cm depending on the pump) 34, distance from the water surface to avoid the risk of sucking in air bubbles 34, and distance from the bottom to avoid stirring up sand.29 Ideally, the pumps should be three-dimensionally alignable.29 The growing trend towards controllable flow pumps 29, which can produce variable patterns such as waves or pulses and often allow for nighttime drawdown, reflects the understanding that mimicking the dynamic flow patterns of natural reefs is beneficial.32 Constant, laminar flow is increasingly viewed as less ideal, justifying investment in more advanced fluid mechanics to create optimal conditions.30

1. Coral aggression: mechanisms and management

In the limited resource of space on a coral reef – and also in an aquarium – there is intense competition.37 Corals have developed sophisticated strategies to assert themselves against neighbors and conquer space to grow.37 This aggression manifests itself in various ways:

• Stinging cells (nematocysts): Microscopic capsules in the tentacles that inject toxins when touched.25 The stinging power varies greatly between species.31 LPS corals often have stronger stinging power than SPS corals.5 Well-known examples of strongly stinging corals are Hydnophora, Acanthastrea echinata 37, Galaxea 39, Euphyllia 14 as well as Catalaphyllia and Plerogyra.43
• Sweeper Tentacles: Specially elongated tentacles with a high concentration of stinging cells at the tip.45 They are used for attacking from a distance and can reach considerable lengths – in Galaxea over 20 cm 43, in Euphyllia up to 15-30 cm (6-12 inches).14 These are typical for many LPS genera such as Euphyllia, Galaxea, Echinopora, Platygyra, Goniopora 37 and Favia 42, but also occur in some PLCs such as Pavona.45
• Mesenteric filaments: Some corals can extend their digestive filaments and place them on neighboring corals to digest them.45 This often happens at night and can quickly lead to massive tissue loss in the victim.45 This behavior has been observed in Favia, Favites, Pectinia, Hydnophora and many chalice corals.45
• Aggressive mucus (mucus coat): For some corals, contact with their mucous membrane can cause rapid damage to others.45 Examples include Acanthastrea echinata and certain chalice corals.45
• Chemical warfare (allelopathy): The release of toxic chemical compounds (e.g. terpenoids, steroids) into water to inhibit the growth, health or survival of competing organisms.35 In the closed system of an aquarium, these toxins can accumulate and cause problems.48 This behavior is particularly well known from soft corals (genera such as Sarcophyton, Lobophytum, Sinularia, Lemnalia 46), gorgonians 50, but also from algae 47, sponges and anemones 46 known. Some stony corals such as Goniopora may also be capable of this.53

The Management of these aggressions is crucial for success, especially in densely populated or mixed reef aquariums:

• Distance: The most important factor is to leave enough space between potentially competing corals.1 The “6-inch rule” (approx. 15 cm) for Euphyllia is a guideline 14, but the tentacles can be longer.14 Galaxea requires particularly large distances.37
• Placement relative to the flow: Aggressive corals can be positioned so that the current directs their fighting tentacles into the open water rather than onto neighbors.37
• Secure positioning: Strongly stinging corals should be placed in such a way that other corals (e.g. when detaching from the substrate) cannot fall into them.37
• Management of allelopathy: In tanks with a high proportion of soft corals or when there are signs of chemical inhibition (e.g. poor growth of SPS despite good water parameters), regular, larger water changes, the use of high-quality activated carbon and/or ozonation can help to reduce toxin levels.45
• Observation: Regular monitoring, including at night (when fighting tentacles and mesenteric filaments are often active), is important to detect conflicts early.45

The diverse aggression mechanisms – from direct physical contact to indirect chemical influence 14 – illustrate that mere spacing is not always sufficient. Especially in mixed tanks containing soft corals, in addition to strategic placement (spacing, flow orientation), the potential problem of allelopathy must also be addressed through appropriate water management (carbon, ozone, water changes).45 The observation that fast-growing corals are often among the more aggressive 39, is evolutionarily logical – they need to secure space for their expansion. For the aquarist, however, this means that the corals that are supposed to fill the tank quickly (e.g. Xenia 37, some Montipora 37), also have the highest potential for conflict and require particularly forward-looking placement with generous buffer zones.35

1. Compatibility and coexistence of different coral groups

Understanding the fundamental differences between the main groups of corals – SPS, LPS and soft corals – is essential for their successful socialization.

1. Small Polyp Stony (SPS) corals

SPS corals (Small Polyp Stony) are characterized by their small polyps and the structure of a solid calcareous skeleton.12 They often form complex, branched or table-shaped structures and frequently dominate the shallow, brightly lit zones of natural reefs, the so-called “reef roof”.12 Their husbandry requirements are demanding: they require high light intensity (typically PAR 200-450 µmol/m²/s or more) 5, a strong, often turbulent water current 5 and very stable water values ​​with low nutrient concentrations (e.g. nitrate < 20 ppm, phosphate < 0.1 ppm).5 They are sensitive to fluctuations in water parameters and temperature.5 A constant supply of calcium, carbonate hardness (alkalinity) and magnesium is essential for the development of their calcareous skeleton.5 Their aggressiveness is considered to be lower compared to LPS; they usually do not have long fighting tentacles and have weaker stinging power.5 However, they are sensitive to the aggression of other corals and should not be placed within reach of heavily stinging neighbors.5 At very low nutrient levels, they can be overgrown by more competitive soft corals.57 However, some species such as Hydnophora can also attack physically.46 Due to their high light and flow requirements, SPS corals are typically placed in the upper part of the reef structure.5 It must be taken into account that their growth can shade underlying areas.37

1. Large Polyp Stony (LPS) corals

LPS corals (Large Polyp Stony), as the name suggests, have large, often fleshy polyps and also form a calcareous skeleton.5 They exhibit a tremendous variety of growth forms. Compared to SPS, their environmental requirements are often more moderate. They generally prefer low to medium light intensity (PAR 50-250 µmol/m²/s). 1 and a moderate to weak, ideally indirect water flow.1 Too strong a current can damage your tissue.14 LPS corals are considered more tolerant to slightly elevated nutrient concentrations and fluctuations in water values ​​than SPS.5 They also need calcium, carbonate hardness and magnesium for their skeletal growth.5 Many LPS species benefit from direct feeding with particulate food (e.g. Mysis, Artemia, special pellets).3 Their aggressiveness is generally higher than that of SPS. They often have strong stinging power and frequently develop long fighting tentacles. 5, or use mesenteric filaments for defense or attack.45 Therefore, they usually require a significant safety distance from other corals.14 A notable exception is the genus Caulastrea, which has only very short tentacles.42 Interestingly, many Euphyllia species (hammer, frogspawn, octopus corals) are compatible with each other, with the exception of the more aggressive torch corals.14 Due to their light and flow preferences, LPS are usually placed in the middle to lower part of the aquarium, often near the bottom on rocks.1

1. soft corals

Soft corals form a very heterogeneous group whose common feature is the absence of a massive calcareous skeleton (although many species incorporate small calcareous needles, called sclerites, into their tissue for support).31 Their requirements are often less specific than those of stony corals. Many species can survive with lower light intensity (PAR 25-150 µmol/m²/s). 1, but there are exceptions here too, and the range is wide.5 The same applies to the current: preferences range from weak to strong.1 A big advantage, especially for beginners, is their high tolerance to fluctuations in water values ​​and higher nutrient concentrations.5 However, some species grow poorly at extremely low nutrient levels.57 Their stinging power is usually low. The main problem with keeping them together is their often rapid, sometimes invasive growth, which can overgrow other corals. 37, and in their ability to allelopathy, i.e. the release of inhibitory or toxic substances into the water.45 Examples of potentially invasive species include xenias, briareum (green star polyps) and various encrusting anemones.37 Many leather corals (Sarcophyton, Sinularia, Lobophytum) and gorgonians are considered to be strongly allelopathic.48 The placement of soft corals is flexible, but often occurs in lower to middle zones.1 For invasive species, placement on isolated rocks (“coral islands”) is recommended to control their spread.37 Due to the potential for overgrowth and allelopathy, it is advisable to keep a distance from sensitive stony corals.

1. Strategies for socialization in the aquarium

The successful joint stance of these different groups requires strategic planning:

• Zoning: The reef structure should be used consciously to create areas with different light and flow regimes.36 Traditionally, SPS are placed on top, LPS in the middle or bottom, and soft corals according to their specific needs.
• Buffer zones: Sufficient free space must be planned between aggressive corals (especially LPS with fighting tentacles) and their potential victims.14 Future growth must be taken into account.1
• Compatible neighbors: More peaceful species can be kept closer together. Knowledge of compatibility within genera (e.g., Euphyllia 14, Zoanthus 37) is helpful.
• Control of invasive species: Fast-growing, over-encroaching soft corals should be placed on separate stones 37 and should be cut back regularly if necessary.
• Management of allelopathy: If there is a significant proportion of soft corals in the tank, the regular use of activated carbon and/or ozone as well as sufficient water changes is advisable to limit the accumulation of toxins.45
• Observation: Continuous monitoring of the tank for signs of nettling, overgrowth or growth inhibition is essential.4

The socialization of SPS, LPS and soft corals is a particular challenge. It requires precise three-dimensional planning that takes into account the often contradictory needs (e.g. light: high vs. low 12; Current: strong vs. weak 12) and the different aggression mechanisms (stinging/tentacles vs. allelopathy 5) are taken into account. It is a balancing act that can force compromises in water values 23 and without a well-thought-out aquascaping and potentially technical management to control allelopathy 45 can hardly be implemented successfully.

III. Comparison of tank concepts: mixed tanks vs. species-dominant systems

The fundamental decision of whether an aquarium should be operated as a mixed tank with a wide variety of corals or as a species-dominant tank specialized in a particular group has profound effects on placement strategies, maintenance effort, and achievable results.

1. The mixed reef: challenges and opportunities in placement

By definition, a mixed reef hosts different coral groups – typically SPS, LPS and soft corals – in a common system.23 The appeal often lies in the great aesthetic variety of shapes and colors.

The challenges are, however, considerable:

• Different needs: It is difficult to create optimal conditions in terms of light, flow and water values ​​for all groups at the same time.23 Compromises often have to be made. A common conflict arises with nutrient levels: While colorful SPS corals, especially Acropora, require extremely low nutrient conditions 12, many LPS and soft corals tolerate or prefer slightly elevated values.42 A nutrient-poor environment optimized for SPS can cause soft corals to decline 57, while more nutrient-rich water will fade the colors of SPS or inhibit their growth.12
• Aggression and competition: The potential for conflicts due to direct entanglement, fighting tentacles and indirect allelopathy is naturally high in a mixed tank.14 This requires extremely careful placement with sufficient spacing and buffer zones.23 Wandering anemones pose an additional, difficult to calculate risk.61
• Maintenance effort: Water parameter management and feeding are more complex because different needs must be taken into account. Supply systems must be able to cover a wide range.23

In contrast they stand opportunities:

• Aesthetic diversity: A mixed tank allows the combination of a wide range of colors, growth forms and textures, which can lead to very varied and natural reef landscapes.58
• Learning potential: It offers the opportunity to gain experience in caring for different types of corals.

Placement strategies in the mixed tank must take heterogeneity into account: strict zoning of the reef structure according to light and current intensity is essential.36 The entire three-dimensional structure must be utilized. Buffer zones between aggressive species must be planned. Selecting less aggressive members of each group can reduce conflict. If the proportion of soft corals is high, allelopathy management with carbon or ozone should be considered.45

1. Species-dominant tanks (e.g. SPS or LPS dominant): Advantages for specialized care and placement

Species-dominant tanks focus on maintaining one main group of corals, for example predominantly SPS or predominantly LPS.

The Advantages The advantages of this approach are obvious:

• Optimized conditions: Light, current and water parameters can be precisely adjusted to the specific needs of the dominant coral group.5 In an SPS tank, for example, very low-nutrient conditions, high light intensities and strong currents can be realized, which are necessary for the optimal growth and coloration of many Acropora species.12 An LPS-dominated tank, on the other hand, can be operated with more moderate light and flow conditions and fed specifically.24
• Potentially reduced aggression: Interactions within the same coral group are often less problematic than between different groups.5 Many SPS species rarely fight each other. With LPS, the situation is more complex, as strong aggression can also occur. 42However, certain combinations, such as a “Euphyllia Garden” (without torch corals), are quite possible.14 The problem of allelopathy caused by soft corals is largely eliminated.
• Simplified placement (relative): Because the basic requirements of corals are more similar, fewer compromises need to be made in placement. More focus can be placed on the individual differences between species or morphs and their growth forms.
• Specialized care: Supply systems and feeds can be used that are specifically tailored to the needs of the coral group being kept.35

The Disadvantages are primarily aesthetic in nature:

• Less diversity: The appearance may be less varied than in a mixed tank.
• Limited selection: One deliberately limits oneself to one group of corals.

Placement strategies in species-dominant tanks Focus on fine-tuning the position based on the specific light and flow preferences of the individual species and color variants within the group. Considering growth form and speed is particularly important here to avoid mutual shading and overgrowth.

The choice between a mixed tank and a species-dominant tank therefore represents a decisive decision. Mixed tanks attract with diversity, but require a high degree of planning, willingness to compromise, and active management of complex interactions.23 Species-dominant tanks allow for specialization and optimization of housing conditions, which often leads to superior results in terms of growth and coloration within the selected group 5, but at the expense of biodiversity in the tank. The decision ultimately depends on the priorities and experience of the aquarist. The observed increase in the popularity of LPS-dominated tanks 24 This could be due to more than just aesthetic reasons. Since LPS generally emits less light 5 and often less current 5 require sophisticated PLC 5 and are also more tolerant to slight increases in nutrients 5, they may be perceived as more energy-efficient and less demanding in terms of nutrient control. This makes them an attractive alternative in times of rising energy prices and for aquarists who do not seek extremely nutrient-poor conditions.

1. Detailed placement recommendations for commonly kept coral genera

The following recommendations provide specific guidance for the placement of common coral species. However, it is important to emphasize that these are guidelines. Individual needs may vary depending on the specific species, color variation (morph), and coral origin (e.g., wild-caught vs. captive-bred, native depth). Careful observation of the coral in your aquarium and its response to the chosen location (polyp pattern, color, growth) remains the key factor for long-term success.4

1. Acropora & Montipora (SPS)

• Acropora: This genus includes many of the most demanding, but also most colorful SPS corals. They require a location in the High-light zone, typically in the upper third of the reef structure, with PAR values ​​of 200 to over 700 µmol/m²/s.10 Find here an strong, turbulent water current is also essential.37 Acropora are very sensitive to fluctuations in water parameters and prefer stable, nutrient-poor conditions.12 Their stinging power is low, but they are vulnerable to the aggression of other corals.5 When placing your often branched or table-shaped growth form and their rapid growth must be taken into account to avoid shading of underlying corals.37
• Montipora: This genus is often somewhat more robust than Acropora, but also prefers a lot of light (PAR 100-400 µmol/m²/s) and strong current.10 Many species grow flat or encrusting (e.g. Montipora plates) 30 They are therefore well suited to colonizing back walls, overhangs, or mid-reef areas. They can be sensitive to sediment deposits, which should be removed regularly.37 Their aggressiveness is low.

1. Euphyllia (LPS – hammer, torch, frogspawn, octopus corals)

• General: Prefer Euphyllia species moderate light (PAR 100-250 µmol/m²/s).14 Torch corals (Torch Corals, E. glabrescens) tend to tolerate slightly higher light intensities.17 The flow should moderate, indirect and ideally alternating or pulsating .14 Too strong or direct current leads to the polyps being drawn in and can damage the tissue.14
• Aggressiveness: Euphyllia are known for their high aggressiveness by long fighting tentacles that can reach lengths of 15 cm (6 inches) to over 30 cm (12 inches).14 A large safety distance of at least 15 cm to others Coral species is mandatory.14
• Internal compatibility: A remarkable feature is the relative tolerability within the (newer taxonomic) genus Fimbriaphyllia, to the hammer corals (F. ancora, F. paraancora), frogspawn corals (F. divisa) and octopus corals (F. paradivisa). These can often be placed close together, allowing the creation of "Euphyllia gardens."14 Torch corals (Euphyllia glabrescens) are an exception here: They are much more aggressive and should not in direct contact with Fimbriaphylliaspecies are kept.17 The rare Grape Coral (Euphyllia cristata) behaves similarly aggressively as torch corals.17
• Placement: Due to their light and flow preferences, Euphyllia are well suited for middle to lower range of the reef, often on ledges or on the bottom on separate stones.14 Sufficient space must be provided for the long tentacles to expand. Care should be taken when handling the polyps to avoid damaging their skeletons; transferring them overhead is recommended.27 Euphyllia are susceptible to bacterial infections such as Brown Jelly Disease.15

1. Caulastrea & Favia (LPS)

• Caulastrea: This genus, often called trumpet or flute coral, is ideal for Low-light zone and close to the ground.37 She prefers low flow and is considered a “flow muffle”.37 A big advantage is theirs low aggressiveness due to very short tentacles, which allows placement closer to other corals.42 It is considered robust and is well suited for beginners and nano aquariums.42
• Favia: Brain corals of the genus Favia require moderate light and flow conditions.42 Variants with intense colors can often tolerate a little more light.42 They are relatively tolerant to nutrients.42 However, Favia corals are aggressive. and can use long fighting tentacles and mesenteric filaments.42 A sufficient safety distance from neighbors is therefore important. They often grow massive or encrusting.30

1. Zoanthus & Palythoa (crust anemones)

• Zoanthus: Zoanthids of this genus are extremely variable in their light requirements, but often thrive in moderate to bright lighting (Guideline value PAR 80-150 µmol/m²/s).10 they prefer moderate currentTheir main feature is that they often rapid, extensive growththat can become invasive.37 Therefore, it is recommended to separate stones (“islands”) to control their spread 37, or to use them specifically as ground cover in the lower reef area.54 Their stinging power is low, but overgrowth of other sessile animals is the main problem.50 Different Zoanthus varieties can easily grow side by side.37
• Palythoa: The keeping conditions are similar to those of Zoanthus. Extreme caution However, it is necessary: ​​Many Palythoa species contain the highly potent neurotoxin Palytoxin.31 Any handling, especially fragmentation or removal of stones, requires mandatory protective measures (gloves, safety goggles) to avoid serious poisoning!

1. Ricordea & Discosoma (disc anemones)

• General: Disc anemones are typical inhabitants of the Low-light zone and are well suited for the bottom or shadier areas in the lower reef.1 Your PAR requirement is approximately 50-150 µmol/m²/s.10 they prefer very low flow and are considered “flow muffles”.34 They reproduce by division and can spread over time.31 Their aggressiveness is low 50, but direct contact can irritate sensitive corals.

1. Other relevant genres (examples)

• Stylophora: Similar to Acropora, they require strong light (PAR 100-550 µmol/m²/s) and strong current and should be placed in the upper reef area.10 They are often considered to be somewhat more robust than Acropora.12
• Goniopora: Daisy corals require a lot of space due to their long polyps and potential fighting tentacles. enough space.37 The light requirement is moderate (PAR 100-300 µmol/m²/s).10 They are often considered demanding in long-term maintenance and may require targeted feeding or special trace elements. Some sources suggest possible allelopathic potential.53
• Galaxea: This genus is extremely aggressive and known for their very long fighting tentacles (over 20 cm) and their strong stinging power.37 It requires a very large safety distance (at least 20 cm is recommended 42), since hardly any other coral survives direct contact.43 Due to its aggressive potential, it is more suitable for larger aquariums.42
• Sarcophyton (mushroom leather coral): These soft corals often prefer well-circulated locations 37 and moderate light (PAR 150-400 µmol/m²/s).10 They can grow very large and dominate the reef structure.54 They are known for their strong allelopathy 46, which must be taken into account when placing in mixing tanks.
• Chalice corals (e.g. Echinopora, Oxypora, Mycedium): These flat-growing LPS often prefer the Low-light zone 37 with PAR values ​​of 100-350 µmol/m²/s.10 Echinopora lamellosa is an exception regarding aggressiveness and can develop long fighting tentacles.37 Some chalice species may also have an aggressive mucosa or use mesenteric filaments.45

The following table summarizes the placement recommendations for the genres discussed:

Table 1: Overview of the placement of commonly kept coral genera in the reef aquarium

genus	Type	PAR (µmol/m²/s)	flow	Aggressiveness	Recommended zone	Notes
Acropora	SPS	200 - 700+	Strong, turbulent	Low (nettle), sensitive to other	Above	Very low in nutrients, stable; consider shading 10
Montipora	SPS	100-400	Stark	Low; patchy species can overgrow	Top / Middle / Wall	Often more robust than Acropora; avoids sediment; flat species for walls 10
Euphyllia (total)	LPS	100-250	Moderate, indirect, variable	High (Fighting tentacles 15-30+ cm)	Middle / Bottom	Large distance (15+ cm) to others genres! 14
-> Hammer/Frog/Octo	LPS	100 – 200 (approx.)	Moderate, indirect, variable	Often compatible with each other	Middle / Bottom	Can form “gardens” 14
-> Torch	LPS	125 – 250 (approx.)	Moderate-strong, indirect	Very high, also compared to other Euphyllia/Fimbriaphyllia	Middle / Bottom	Nicht mix with Hammer/Frog/Octo! 17
Caulastrea	LPS	< 100 (approx.)	Weak	Low (short tentacles)	Bottom / Floor	Good for beginners, Nanos; can be placed closer 37
Favia	LPS	100 – 200 (approx.)	Moderate	High (fighting tentacles, mesenteric filaments)	Middle / Bottom	Distance necessary; more tolerant to nutrients 42
Zoanthus	crust	80 - 150+	Moderate	Low (Nettle), but invasive growth (overgrowth)	Bottom / Ground / Islands	Set on islands for control; compatible with each other 10
Palythoa	crust	80 - 150+	Moderate	Low (nettle), invasive growth; HIGHLY TOXIC (Palytoxin!)	Bottom / Ground / Islands	Be careful when handling! Protective measures necessary! 31
Ricordea/Discosoma	slices	50-150	Weak	Low (Contact irritation possible), reproduction by division	Bottom / Floor	“Flow muffle” 10
Stylophora	SPS	100-550	Stark	Low (Nettle)	Above	Similar to Acropora, often more robust 10
Goniopora	LPS/SPS-near	100-300	Moderate	High (fighting tentacles); potentially allelopathy	Middle / Bottom	Space requirements; often demanding (food) 10
Galaxea	LPS	100 – 200 (approx.)	Moderate	Extremely High (very long tentacles >20cm, strong stinging)	Middle (with plenty of space)	Very large distance (20+ cm!) necessary; hardly tolerated by neighbors 37
sarcophyton	Soft	150-400	Moderate to severe	Allelopathy; can grow very large	Middle / Top	Consider chemical effects on other corals 10
Chalice (general)	LPS	100-350	Mild to moderate	Variable; some with fighting tentacles (Echinopora), mesenteric filaments, mucus	Bottom / Middle	Note specific species; Echinopora lamellosa aggressive 10

It is clear that even within established species there can be considerable differences in needs and behavior.17 Not all Favias are created equal, and not all Euphyllias are created equal. This underscores the importance of knowing not only the genus, but also the specific species and, if applicable, even the color variant, before placing a plant.37 General genre recommendations serve as valuable starting points, but do not replace individual research and observation.4

1. Acclimatizing new corals: A stress-free start

Introducing new corals into an established reef aquarium is a critical moment that can determine the animal's future well-being. Careful acclimation minimizes stress and significantly increases the chances of survival. The process includes adaptation to the water parameters, potential anti-parasitic treatment, and gradual acclimation to the lighting conditions.

1. Acclimatization to water parameters and temperature

The first step is this Temperature adjustmentTo do this, the closed transport bag is placed in the aquarium water for about 15 to 20 minutes so that the water temperature in the bag can adjust to that of the aquarium.27

The need for a slow Adjustment of water parameters (especially salinity/density, pH, alkalinity) is evaluated differently for corals.

• The Droplet method, in which aquarium water is slowly dripped into the transport bag or a separate container over a period of approximately 30 minutes, is the standard method for sensitive invertebrates such as shrimp, mussels or anemones.27 The amount of water in the bag is slowly increased and partially discarded in between in order to gradually adjust the parameters.27
• For corals, some sources argue that such a slow adjustment is not necessary.64 Recommendations range from direct transfer after temperature adjustment 64 by adding some aquarium water for 20 minutes 64 up to letting them swim briefly in a bucket of aquarium water for 15-30 minutes.28 The main reason for rapid transfer in fish (accumulation of ammonia in the transport water through respiration) is less critical in corals.27
• As a Suggestions A middle ground seems sensible: While robust corals may be able to cope with a quicker transfer, for more sensitive species or in cases of significant differences in water values ​​(especially salinity), a careful adjustment over 15 to 30 minutes is advisable in order to avoid additional stress.28

1. Coral dips: necessity, methods and products

A coral dip is a bath in a special solution outside the aquarium, which serves to remove potential Parasiten (such as Montipora-eating snails, Zoanthus-eating spiders, Acropora-eating flatworms – AEFW), pests (e.g. bristle worms) and other undesirable Accompanying fauna or deposits from newly acquired corals before they enter the main tank.4 At the same time, it is intended to strengthen the resilience of the coral.31

Performing a dip is strongly recommended, as the risk of introducing new coral pests or diseases into the aquarium is considerable.4 An alternative would be a separate quarantine tank in which new corals can be observed over several weeks.4 However, since this is not practical for many aquarists, dipping is an important preventative step.

There are different Products on the market, e.g. Fauna Marin THE DIP 4, Polyplab Reef Primer 70, Coral RX 68 or Coral Breeding Ultra Pest Control.67 Some aquarists also use iodine solutions (e.g. PVP-iodine) 68 or carefully dosed hydrogen peroxide (H₂O₂).68 The latter, however, requires experience, as tolerance depends heavily on the coral species and concentration.68

The Application is similar for most commercial products (example: THE DIP): A defined amount of the concentrate (e.g., 15 ml) is mixed with 1 liter of aquarium water in a separate container. The coral is then placed in this bath for a short time (usually a maximum of 5 minutes) and gently agitated. The coral is then briefly rinsed in another container with clean aquarium water before being placed in the aquarium. The bathing and rinsing solutions are then discarded.66

Important hints: Coral dips may never in the aquarium be applied.66 They are not suitable for fish, shrimp, mussels, sea urchins or similar animals; these must be removed from the coral before bathing.66 When handling, gloves should be worn and contact with the eyes should be avoided.66 You have to be aware that a dip can kill not only pests, but also beneficial microorganisms (such as small brittle stars, amphipods, copepods) on the coral.68 In addition, the bath always means a certain amount of stress for the coral itself.68

1. Light acclimatization and finding the final position

After adjusting to the water parameters and the potential dip, the last but crucial step of acclimatization follows: the Light acclimatizationAs already explained in Section IA, new corals should not be placed immediately in the brightest spot intended for them, as this can lead to light stress and bleaching.8

The Strategy consists in first attaching the coral to a shadier place to be placed in the aquarium, for example near the bottom or under an overhang.8 From there, it will be several days to several weeks gradually accustomed to their final location with the appropriate light intensity.8

The Finding the final position Requires observation skills. You have to pay attention to how the coral reacts to the respective location: Is it showing good polyp expansion? Is its color remaining stable or is it lightening/darkening?4 Once a position has been found where the coral appears to be comfortable (good polyp pattern, stable color), it should be left there.14 Frequent relocation should be avoided as corals prefer stability.4

Only when the final position has been found and the coral has acclimatized, it should firmly connected to the reef structure Special coral adhesives (cyanoacrylate gel) or two-component epoxy putties are suitable for this purpose.2 The “sandwich method,” in which a small dab of superglue is applied, followed by a piece of mixed epoxy putty and then another dot of superglue on the fragstein, has proven to be effective for a quick and lasting hold.2 The often present stem of the fragstein (“plug”) can be removed beforehand with a side cutter to allow for a flatter support.2 When handling, care should be taken to only touch the stone or skeleton and not the delicate coral tissue.27

The entire acclimatization process is a balancing act. It is important to minimize the stress caused by transport and parameter changes 27 while reducing the high risk of introducing pests and diseases.4 While temperature equalization is undisputed, opinions differ regarding the slow water equalization in corals.28 Dipping is widely recommended 66, but carries its own risks such as additional stress and the killing of beneficial microfauna.68 There is no universally perfect method; the optimal strategy depends on the robustness of the coral species, the quality of the source and the possibilities of the aquarist (e.g. availability of a quarantine tank 4) and requires an individual risk assessment. The subsequent light acclimatization 8 is not only a short-term protection against bleaching, but lays the foundation for the long-term establishment and prosperity of the coral in its final location by enabling optimal physiological adaptation to the local light conditions.4

1. Long-term perspectives: growth and basin dynamics

Placing a coral is not the end, but the beginning of a dynamic process. Corals are living organisms that grow and, in turn, change the structure and conditions in the aquarium over time. Forward-looking planning that takes this growth into account is crucial to ensuring long-term harmony in the reef tank.

1. Consider the growth potential of different corals

Corals have very different growth rates .4 Some SPS species (e.g., many Acropora, Montipora, Stylophora) can grow very rapidly under optimal conditions. However, certain LPS species (e.g., Euphyllia) and, in particular, some soft corals (e.g., Xenias, Clavularia, Capnella) can also spread rapidly.37 However, the actual growth rate depends strongly on the housing conditions: light intensity and spectrum, current, water quality (nutrients, trace elements) and food supply play an essential role.4

It is equally important to consider the growth habit. If a coral grows branches in height and width (e.g. many Acropora 12)? Does it form flat plates or crusts (e.g. Montipora plates, Chalice corals, Cyphastrea, Leptastrea 30)? Or does it grow more spherically or massively (e.g. Favia 30)? The growth form largely determines how much space a coral will take up and how it interacts with its environment (e.g. shading by plate corals 37).

This results in the need to consider the initial placement of the future space requirements plan generously.1 A newly set up aquarium may initially appear sparsely populated, but fast-growing corals can quickly fill this space.2 There must be enough space between each coral, not only because of aggression, but also to allow room for growth.1

1. Strategies to avoid future conflicts through growth

To prevent growing corals from crowding, entangling, overgrowing or shading each other, the following strategies are helpful:

• Proactive placement: Position corals from the outset to take their expected growth into account.1 Aggressive species require correspondingly larger buffer zones.14
• Anticipate growth direction: Try to estimate in which direction a coral will primarily expand and take this into account when positioning it relative to neighbors and currents.
• Regular pruning (fragging): For many fast-growing species, it is essential to trim them regularly to control their spread and make room for other corals.21 The fragments obtained can be used for reproduction or exchange.31
• Adaptability and flexibility: Even with the best planning, corals may need to be relocated if unforeseen conflicts arise. Although corals prefer stability 4, intervention is sometimes necessary. A fastening method that allows for later removal (e.g., gluing to small base stones rather than directly to the main rock) can be advantageous in this case.2
• Acceptance of the dynamics: A reef aquarium is not a static painting, but a constantly evolving ecosystem.75 Coral growth inevitably changes the structure of the reef, which in turn changes the current patterns 21 and influences the light distribution in the pool.37 In the long term, adjustments and interventions by the aquarist are therefore often unavoidable.

The placement of corals is therefore not a one-time act, but the starting point of a dynamic process. Coral growth continuously reshapes the reef. Growing colonies alter hydrodynamics by blocking or redirecting flow paths. 21, and influence the light availability for lower areas.37 Growing corals can also invade the habitat of their neighbors and trigger new conflicts through entanglement or overgrowth.1 Successful long-term care therefore requires not only a well-thought-out initial planning that takes into account the growth potential and the growth form 1, but also the willingness to continuously monitor and actively manage through targeted pruning 21 or occasional relocation over the entire lifespan of the aquarium.

VII. Aesthetics meets biology: Aquascaping with corals

Aquascaping in saltwater aquariums, often referred to as “reefscaping,” is the art of creating an underwater landscape that is not only aesthetically pleasing but also meets the complex biological needs of the corals and other reef inhabitants being cared for.59 It's about creating a harmonious and functional ecosystem.

1. Design principles for a harmonious reef image

Although biological needs should be paramount, principles from art and freshwater aquascaping can help create a more visually appealing reef:

• Planning: As with any design process, careful planning is essential. Visualizing and possibly sketching the desired layout will help avoid mistakes.60 The coral species to be cared for, their growth forms, colors and needs must be taken into account from the outset.60
• Rule of thirds / Golden ratio: These classic design rules can help achieve a more balanced and dynamic composition. Important elements or focal points (e.g., a particularly magnificent coral, a striking rock formation) are not placed in the center, but rather along imaginary lines that divide the aquarium horizontally and vertically into thirds, or at their intersections.1
• Creating depth: Arranging the reef structures in multiple levels rather than a single line creates an illusion of depth.59 Varying heights and depths in the design, as well as deliberately leaving space, contribute to this. A deeper pool (e.g., 70 cm or more) facilitates implementation.59 A clean back wall enhances the impression of depth.59
• Contrasts and color design: The deliberate play with contrasts in colors, shapes (e.g. branchy next to solid) and textures of the corals can be very effective.59 Sometimes less is more: A conscious restriction to a few harmonious color themes can have a stronger effect than an overloaded “candy shop.”59
• Negative space / free space: Not every inch of the aquarium needs to be filled with rock or coral. Deliberately leaving open sand areas or swimming areas for fish can structure the layout and better showcase the reef structures.59 Corals should not be placed directly on the sand, but ideally on small base stones to appear more natural.59
• Naturalness and technology integration: The reef structure should appear as natural as possible, without artificial edges, plates or piles.59 Visible technology (pumps, heaters, overflows) should be hidden or integrated as cleverly as possible.59
• Focus point: Selecting one or a few main corals or a striking structural group as an eye-catcher can give the layout stability.1

1. Integration of biological needs into the aesthetic concept

The crucial difference to purely decorative design lies in the need to integrate the biological requirements of the corals into the design:

• Functional scaping: The reef structure is not just decoration, but it creates the habitats. It must be designed to create the necessary zones for different light intensities and flow velocities.37 The corals are then placed according to their needs within this functional framework.
• Plan for growth: The design must provide space for future coral growth from the outset to minimize later conflicts due to overgrowth or shading.37
• Accessibility of care: An aesthetically pleasing but inaccessible reef is unsustainable in the long term. The structure must be designed to ensure that all areas remain accessible for cleaning (e.g., window cleaning, vacuuming detritus) and maintenance of the equipment.76 Removable reef elements or corals on separate “islands” can make maintenance easier.37

1. Practical techniques and tools for fastening and arrangement

Various materials and tools are available to implement the planned reef structure and to secure the corals:

• Fastening materials: Standard are special cyanoacrylate-based coral adhesives (gel form) and 2-component epoxy putties.2 The combination of both materials (“sandwich method”) often offers the best compromise between fast initial hold (adhesive) and permanent stability (epoxy).2 Reef mortar is suitable for the construction of larger, permanent structures.
• question stones and plugs: Coral fragments are usually delivered on standardized ceramic or cement plugs.2 These can either be glued directly onto the reef structure or inserted into pre-drilled holes.2 The stem of the plug is often removed to achieve a flatter support.2
• aids: Light grid plates made of plastic can serve as a stable base for the reef structure (protection of the bottom plate), as a base for question frames or for structuring filter compartments.71 Magnetic coral holders (frag racks) allow flexible placement of frags on the aquarium panes or in corners without changing the main structure.71
• DIY structures: Individual and airy reef structures can be created using reef ceramics, dead reef rock or special aquascaping stones.71

Successful reef aquascaping is therefore the challenging task of creating aesthetic visions 1 with the complex biological realities of coral care (light, currents, competition for space, growth [Sections I, II, VI]). The result is an environment that is both beautiful and functional, allowing the corals to thrive long-term. This can lead to conflicting objectives—the biologically optimal position is not always the aesthetically desired one—that require creative compromises and solutions. Modern aquascaping approaches in marine water, such as airy column structures, overhangs, or "negative scaping" (inward-sloping structures) 76, are increasingly moving away from the traditional, solid "reef wall." This trend is not only aesthetically motivated, but also offers clear functional advantages: improved circulation throughout the entire reef body, avoidance of detritus traps, easier cleaning, and more diverse, targeted placement options for corals with different requirements.76 This reflects a deeper understanding of the hydrodynamic and ecological processes in the closed aquarium system and leads to designs that intelligently combine beauty and function.

Conclusion

Optimally placing corals in a saltwater aquarium is a complex challenge that goes far beyond simply arranging decorations. It requires a thorough understanding of the basic biological needs of each individual coral as well as the complex interactions within the artificial reef ecosystem.

The key factors of light (intensity/PAR and spectrum), water current (strength and type), and aggression potential (stinging, fighting tentacles, allelopathy) must be individually assessed for each coral and taken into account when choosing a location. Knowledge of the specific requirements of common coral groups—the light-hungry and current-loving SPS, the often more frugal but more aggressive LPS, and the diverse, sometimes invasive or allelopathic soft corals—is fundamental for successful socialization.

The decision between a diverse but challenging mixed tank and a specialized, more easily optimized species-dominant tank determines the framework for the placement strategy. Regardless of the concept, a careful, stress-minimizing acclimation of new corals, ideally including a preventative dip and gradual light acclimation, is essential for a successful start in the new location.

In the long term, the dynamic growth of corals must be planned for. Forward-looking placement that anticipates future space requirements and growth habit, as well as a willingness to actively manage through pruning or relocation, are necessary to avoid conflicts caused by overgrowth, shading, or entanglement.

Ultimately, aquascaping combines biological necessity with aesthetic appeal. Thoughtful design principles that emphasize depth, contrast, and naturalness must go hand in hand with the creation of functional zones for light and flow, as well as consideration of growth and maintenance accessibility.

In summary, successful coral placement is a process that requires knowledge, careful planning, patience during acclimation, and continuous observation. The reef aquarium is a living system that is constantly changing and requires adaptability from the caregiver. However, those who heed these principles will be rewarded with a healthy, growing, and aesthetically impressive coral reef.4 It is advisable to pay attention to sustainability when selecting corals and, whenever possible, to use captive-bred corals to protect natural reefs.31

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