Guides / Coral

Reef Coral Guide: Types, Care, Placement, and What

Quick Summary

Corals are the defining organisms in a reef tank. They are not plants and they are not rocks. They are animals, colonial invertebrates that build calcium carbonate skeletons and host photosynthetic symbionts within their tissue. Everything in a reef tank, from lighting to water chemistry to flow, exists to keep corals alive and growing.

If you are starting with corals or expanding your collection, here is what matters most:

Start with forgiving species. Mushrooms, zoanthids, and leather corals tolerate a wide range of conditions and teach you the fundamentals without punishing mistakes.
Placement determines success. The same coral can thrive or die depending on where you put it. Light intensity, flow pattern, and proximity to neighbours all matter.
Stability beats perfection. Corals adapt to a range of parameters but cannot tolerate rapid changes. A stable tank at 8 dKH grows coral better than one swinging between 9 and 11.
Corals are not passive. They sting, compete for space, and wage chemical warfare. Planning your aquascape with compatibility in mind prevents losses.

How Corals Work

Most reefers know that corals need light, but fewer understand why. The relationship between a coral and its internal symbionts is the foundation of everything in reef keeping.

Corals host microscopic dinoflagellates called zooxanthellae (family Symbiodiniaceae) inside their tissue cells. These zooxanthellae photosynthesize, converting light energy into sugars that they share with the coral host. In return, the coral provides shelter, carbon dioxide, and nutrients. This symbiosis provides up to 90 percent of the coral's energy needs.

The coral itself is an animal. It has a mouth, a digestive cavity, and tentacles equipped with stinging cells (nematocysts) for capturing food. Corals can feed heterotrophically by catching plankton and dissolved organic particles from the water. But photosynthesis through zooxanthellae is the primary energy source for most reef tank species.

This is why lighting is so critical. Without appropriate light, zooxanthellae cannot photosynthesize, and the coral slowly starves. Too much light overwhelms the zooxanthellae, causing oxidative stress that leads to bleaching. The balance between sufficient and excessive light is the core challenge of coral husbandry.

The coral's skeleton is built through calcification. The animal extracts calcium and carbonate ions from the water and deposits them as crystalline aragonite beneath its tissue. This process consumes alkalinity and calcium continuously, which is why these parameters require daily monitoring and replenishment in any tank with actively growing stony corals.

The Three Categories of Coral

Reef corals are broadly grouped into three categories based on their skeletal structure and care requirements. Understanding these categories helps you select corals appropriate for your system and experience level.

Soft Corals

Soft corals do not build a rigid calcium carbonate skeleton. Instead, they have a flexible, fleshy body supported by internal spicules (small, needle-like calcium carbonate structures) embedded in their tissue. This gives them a rubbery, organic appearance and texture.

Common soft corals in the hobby:

Mushroom corals (Discosoma, Rhodactis, Ricordea). Among the easiest corals to keep. They prefer low to moderate light and gentle flow. They come in an enormous range of colours and patterns, and some varieties (bounce mushrooms, jawbreaker mushrooms) command high prices despite their beginner-friendly care requirements.
Zoanthids and palythoa. Colonial polyps that encrust across rock surfaces. Extremely hardy, available in hundreds of colour morphs, and one of the most collected coral types in the hobby. Palythoa species contain palytoxin, a potent toxin. Handle with gloves and never boil or frag them in enclosed spaces.
Leather corals (Sinularia, Sarcophyton). Large, tree-like or mushroom-shaped soft corals. Very hardy and fast-growing. They periodically shed a waxy coating, during which they close for several days. This is normal, not a sign of stress.
Green star polyps (GSP). A rapidly encrusting soft coral that forms a purple mat with green polyps. Extremely hardy but aggressive. GSP will overgrow rock, glass, and neighbouring corals if not contained. Many reefers grow it on an isolated rock island to prevent it from taking over the aquascape.
Xenia and pulsing xenia. Known for their rhythmic pulsing motion. Xenia is one of the fastest-growing soft corals and can become invasive. It reproduces by dropping small pieces that attach and grow on new surfaces.
Kenya tree corals (Capnella). Hardy, fast-growing, and self-propagating. Kenya trees drop branches that settle and attach to nearby surfaces, forming new colonies.

Soft corals are the recommended starting point for new reefers. They tolerate parameter fluctuations, recover quickly from stress, and grow fast enough to provide visible progress while you develop your skills.

However, soft corals have a trade-off. Many species, particularly leathers and xenia, release terpenes and other allelopathic chemicals into the water that can suppress or kill stony corals. Running activated carbon helps mitigate this, but reefers planning to transition to SPS-dominant systems should be aware that a tank full of soft corals creates a chemical environment that is hostile to sensitive stony species.

LPS Corals (Large Polyp Stony)

LPS corals build a rigid calcium carbonate skeleton like SPS, but their polyps are significantly larger, often fleshy and inflated during the day. They occupy a middle ground in care requirements between soft corals and SPS.

Common LPS corals:

Euphyllia (torch, hammer, frogspawn). The most popular LPS genus. Long, flowing tentacles that create dramatic movement in the tank. Moderate light (100 to 200 PAR) and moderate flow. Euphyllia species extend sweeper tentacles at night that can reach 6 to 10 inches, stinging anything they touch. Space them accordingly.
Acanthastrea and micromussa (acans). Encrusting corals with fleshy, rounded polyps in vibrant colours. Low to moderate light. They are aggressive feeders and respond well to target feeding.
Brain corals (Lobophyllia, Trachyphyllia). Large, dome-shaped corals with elaborate skeletal patterns. Prefer the sandbed or low rock with moderate light and gentle flow. Trachyphyllia (open brain) is a free-living coral that sits on the sand rather than attaching to rock.
Blastomussa. Clusters of large, round polyps. Very hardy for an LPS, tolerating a range of conditions. Low to moderate light and gentle flow.
Goniopora. Beautiful flower-like polyps that extend several inches from the skeleton. Historically considered difficult, but aquacultured specimens are significantly hardier than wild-collected ones. Requires consistent feeding.
Scolymia and Cynarina. Large, single-polyp corals with vivid colours. Solitary and non-aggressive. Prefer the sandbed or low placement with moderate light.

LPS corals consume alkalinity and calcium but at a lower rate than SPS. They are more forgiving of parameter fluctuations than SPS but less tolerant than soft corals. Most LPS do well with alkalinity held between 7 and 10 dKH.

The primary management challenge with LPS is aggression. Euphyllia sweeper tentacles, acan feeding tentacles, and brain coral mesenterial filaments can all damage neighbouring corals. Spacing and placement planning are essential.

SPS Corals (Small Polyp Stony)

SPS corals are the pinnacle of reef keeping difficulty and reward. They build dense, branching or plating calcium carbonate skeletons covered in tiny polyps. When healthy, they display some of the most vivid colours in the marine world. When stressed, they decline rapidly.

Common SPS corals:

Acropora. The most iconic SPS genus. Branching, tabling, and staghorn growth forms. Requires high light (250 to 450+ PAR), strong turbulent flow, and rock-stable alkalinity. Acropora is the benchmark species for reef tank quality: if Acropora is thriving, everything else in the system is almost certainly fine.
Montipora. Plating, encrusting, and branching forms. More forgiving than Acropora, tolerating moderate light (150 to 300 PAR) and moderate flow. Montipora is often the gateway SPS coral for reefers stepping up from LPS.
Pocillopora. Compact, bushy growth form. Hardy for an SPS, tolerating moderate conditions. Fast-growing and a good SPS starter.
Stylophora. Dense, branching colonies. Similar care requirements to Pocillopora. Responds well to moderate to high light and strong flow.
Seriatopora (birdsnest coral). Fine, delicate branching structure. Moderate to high light. Somewhat brittle but fast-growing.

SPS corals demand the tightest parameter control of any coral category. Alkalinity must be stable within 0.5 dKH day to day. Calcium must be maintained between 400 and 450 ppm. Magnesium between 1300 and 1400 ppm. Nitrate between 2 and 10 ppm. Phosphate between 0.02 and 0.08 ppm.

The margin for error with SPS is small. An alkalinity swing that soft corals would not even notice can cause tissue necrosis in Acropora within 48 hours. This is why SPS corals are recommended only after a tank has been stable for at least 6 months and the reefer has demonstrated consistent parameter maintenance.

Coral Placement Strategy

Where you place a coral in your tank determines whether it thrives, struggles, or dies. Placement involves three factors: light, flow, and proximity to other corals.

Light Zones

Most reef tanks have a natural light gradient from top to bottom. The highest PAR is directly under the light at the top of the aquascape. PAR decreases as you move down and toward the edges.

Use this gradient deliberately:

Top zone (highest PAR). SPS corals, particularly Acropora. Only place species here that demand high light.
Mid zone. LPS corals, Montipora, and Pocillopora. Most corals perform well in this range.
Lower zone and sandbed. Mushrooms, brain corals, Scolymia, open brains. Species that prefer lower light or that are free-living (not attached to rock).
Shaded overhangs. Non-photosynthetic corals (if you keep them), cave-dwelling species, and corals recovering from light stress.

In practice, the most common placement mistake is putting corals too high. A new reefer buys a beautiful LPS, places it at the top of the rockwork under maximum PAR, and watches it bleach within a week. Always start low and move up gradually.

Flow Considerations

Different corals need different flow patterns:

Soft corals generally prefer gentle, indirect flow. Too much flow pins their tissue flat and prevents polyp extension.
LPS corals need moderate flow. Enough to deliver food and exchange gases, but not so much that it prevents polyp expansion. Euphyllia tentacles should sway gently, not be pinned sideways.
SPS corals demand strong, turbulent, randomised flow. SPS polyps are tiny and rely on flow for gas exchange and food delivery. Dead spots around SPS lead to tissue recession and algae growth on the skeleton.

You will often notice that the same coral behaves differently in different flow positions. A torch coral with full, flowing tentacles in moderate flow will retract completely if placed in a direct powerhead stream. Position first, observe, and adjust if the coral is not extending.

Aggression and Spacing

Corals compete for space, and many do so aggressively. Planning spacing to account for aggression prevents losses.

Key aggression types:

Sweeper tentacles. Extended at night by Euphyllia, galaxea, and some brain corals. Can reach 6 to 12 inches and sting anything they contact. Allow at least 6 inches of clearance around species known for sweepers.
Mesenterial filaments. Digestive filaments extruded by some LPS corals that dissolve the tissue of neighbouring corals. Brain corals and favites use these effectively.
Allelopathy. Chemical warfare through compounds released into the water. Leather corals, xenia, and soft corals in general release terpenes that suppress stony coral growth. The effect is system-wide, not just proximity-based. Running activated carbon mitigates but does not eliminate it.
Overgrowth. Fast-growing encrusting corals (GSP, xenia, Montipora) can physically overgrow and smother slower-growing neighbours. Contain fast-growers on isolated rock islands with gaps that prevent bridging.

The general rule is to give every coral more space than you think it needs. Corals grow. A colony that has 4 inches of clearance today may be touching its neighbour in six months.

Feeding Corals

Corals derive most of their energy from photosynthesis, but supplemental feeding improves growth, colour, and overall health, particularly for LPS and non-photosynthetic species.

What to Feed

Effective coral foods:

Reef-specific liquid foods (amino acids, phytoplankton blends). Dose to the tank for broadcast feeding. Benefits filter feeders, soft corals, and SPS.
Frozen or live mysis and brine shrimp. Target feed to LPS corals with large mouths (brain corals, acans, Scolymia). Use a pipette or turkey baster to deliver food directly to polyps.
Coral-specific powdered foods (Reef Roids, Coral Frenzy). Fine particle foods that SPS polyps can capture. Broadcast or target feed.
Live phytoplankton. Supports filter feeders and the broader microbial ecosystem. Dose daily or every other day.

How to Feed

Target feeding is the most efficient method for LPS and large-polyp species. Turn off flow pumps for 10 to 15 minutes, deliver food directly to polyps with a pipette, and observe the feeding response. Most LPS corals close their tentacles around the food and pull it toward their mouth.

For SPS and broadcast feeding, dose liquid or powdered food into the water column with flow running. The particles distribute throughout the tank and SPS polyps capture what they can.

In most tanks, feeding corals 2 to 3 times per week produces noticeable improvements in growth and colour. Daily feeding is beneficial for heavy feeders like Goniopora and non-photosynthetic species.

Do not overfeed. Excess coral food decays and contributes to elevated nitrate and phosphate. Feed amounts that corals consume within 15 to 20 minutes.

Fragging: Propagating Your Corals

Fragging (fragmenting) is the process of cutting or breaking a coral colony to create new pieces that grow into independent colonies. It is how most corals in the hobby are propagated, and it is a normal part of long-term reef keeping.

Why Frag

Share and trade with other reefers.
Save a dying coral by fragging healthy tissue above the recession line.
Control growth of fast-spreading species.
Generate income from selling frags of desirable morphs.

How to Frag by Coral Type

Different coral types require different fragging techniques:

Soft corals: Cut with sharp scissors or a razor blade. Most soft corals heal quickly. Leather corals, mushrooms, and zoanthids can be fragged by cutting a portion and attaching it to a frag plug with superglue gel or rubber band.

LPS corals: Use a band saw, Dremel, or bone cutters to cut through the skeleton. Cut between polyp heads, leaving at least one complete polyp per frag. LPS heals slowly, so minimise handling and place frags in low-flow areas during recovery.

SPS corals: Snap branches at natural branching points or use bone cutters. Acropora and Montipora frag easily. Attach frags to plugs with superglue gel. SPS frags typically encrust the plug within 2 to 4 weeks and begin growing from there.

Fragging Best Practices

Practical advice for successful fragging:

Frag only healthy, growing corals. Fragging a stressed coral adds to its stress.
Use clean, sharp tools. Dull cuts cause more tissue damage and slower healing.
Dip frags in a pest treatment after cutting to prevent introducing hitchhikers.
Place fresh frags in moderate light and flow. Avoid direct high PAR on fresh cuts.
Allow 2 to 4 weeks for recovery before moving frags to their permanent position.

Common Coral Problems

Even in well-maintained tanks, corals encounter problems. Recognising issues early improves outcomes.

Bleaching. Loss of colour (paling to white) indicates zooxanthellae expulsion. Caused by temperature spikes, light shock, or alkalinity swings. Reduce light, stabilise parameters, and wait. See the coral bleaching guide for full details.

Tissue recession. Skeleton becoming visible at the base or edges of a colony. Caused by alkalinity instability, aggression from neighbours, or pests. Identify and remove the cause. If recession is progressing rapidly, frag above the recession line.

Brown jelly disease. A bacterial infection that appears as a brownish, gelatinous mass on tissue. Rapidly fatal. Remove the affected coral immediately, dip it aggressively, and frag any healthy tissue. Do not return the affected area to the display.

Rapid tissue necrosis (RTN). Tissue sloughs off the skeleton in hours, typically starting at the base and progressing upward. Usually triggered by severe alkalinity swings or contamination. Frag above the necrosis line immediately to save what you can.

Slow tissue necrosis (STN). Similar to RTN but progresses over days to weeks. Causes include chronic parameter instability, low alkalinity, and pest damage. More time to intervene than RTN, but the cause must be identified and addressed.

Advanced: Coral Colour and What It Tells You

Coral colour is not just aesthetic. It is diagnostic. The colour of a coral reflects its zooxanthellae density, the types of pigments it produces, and its overall health status.

Bright, vivid colours indicate a healthy balance between zooxanthellae density and the coral's own fluorescent and chromatic pigments. The coral is producing photoprotective proteins (which create blues, purples, and pinks) and the zooxanthellae population is balanced for the available light and nutrients.

Brown colouration indicates excess zooxanthellae density. The coral is "over-zooed." This happens in high-nutrient, moderate-light conditions. The zooxanthellae overpopulate and their brown pigments dominate the coral's appearance. Growth may be normal or slowed, but the coral lacks the vivid colours that balanced conditions produce.

Pale or pastel colours can indicate two things: either the coral is beginning to lose zooxanthellae (early bleaching) or it is in a low-nutrient, high-light environment where zooxanthellae density is sparse but the coral's own pigments are visible. Context determines which interpretation is correct.

White is bleached. The coral has expelled most or all of its zooxanthellae, revealing the white aragonite skeleton through transparent tissue. If tissue is present (smooth white), recovery is possible. If bare skeleton is exposed (rough white), that portion is dead.

Understanding these colour signals helps you read your tank. Brown corals in a new system typically indicate elevated nutrients and a need for better export. Corals shifting from vivid to pale in a mature system may indicate early stress from a parameter shift.

Advanced: The Chemical Environment of a Mixed Reef

Running soft corals, LPS, and SPS in the same tank creates a complex chemical environment. Different coral groups produce different allelopathic compounds, and these compounds interact with each other and with the corals they were not directed at.

Soft corals are the biggest chemical producers. Leather corals release terpenes that can reduce SPS growth rates even at opposite ends of the tank. Xenia and GSP release compounds that suppress competing corals in their immediate vicinity. Zoanthids produce palytoxin, which is primarily a handling hazard but also contributes to the chemical soup of a mixed reef.

LPS corals use mechanical aggression (sweeper tentacles, mesenterial filaments) more than chemical aggression, but some species also release chemical compounds during stress or competition.

SPS corals are generally the least chemically aggressive and the most chemically sensitive. They are the group most likely to suffer in a mixed reef environment dominated by soft corals.

In practice, successful mixed reefs manage this chemical environment through:

Running activated carbon continuously (replaced every 3 to 4 weeks)
Performing regular water changes to dilute chemical concentrations
Maintaining strong skimming to remove dissolved organics that carry allelopathic compounds
Providing adequate space between coral groups
Accepting that SPS growth rates in a mixed reef may be lower than in an SPS-only system

Common Myths

"Corals are plants." Corals are animals. They belong to the phylum Cnidaria, which includes jellyfish and anemones. The photosynthesis happens in the zooxanthellae living inside the coral tissue, not in the coral itself.

"All corals need high light." Many popular corals (mushrooms, zoanthids, many LPS) prefer low to moderate light. Placing them under high PAR causes bleaching and stress. Match light to species requirements, not to a single standard.

"SPS corals are impossible for beginners." SPS corals require stable parameters and patience, but they are not impossible. Montipora and Pocillopora are relatively forgiving SPS species. The key is waiting until your tank is mature (6+ months) and your parameter maintenance is consistent before attempting them.

"Corals do not need to be fed." Photosynthesis provides the majority of energy, but supplemental feeding improves growth, colour, and resilience. LPS corals in particular benefit significantly from target feeding.

"You can mix any corals together." Coral compatibility matters. Mixing aggressive species (Euphyllia, galaxea) with passive species without adequate spacing leads to tissue damage and death. Chemical warfare between soft corals and SPS is real and can suppress growth across the entire tank.

FAQ

What are the best corals for a beginner? Mushroom corals, zoanthids, green star polyps, and leather corals are the most forgiving starter corals. They tolerate a range of conditions and recover quickly from mistakes.

How fast do corals grow? Growth rates vary enormously. Soft corals like xenia and GSP can double in months. SPS corals typically grow 1 to 3 inches per year under optimal conditions. LPS growth is somewhere in between. Stability and consistent parameters matter more than any single supplement or technique.

Can I keep soft corals and SPS together? Yes, but with management. Soft corals release allelopathic compounds that can suppress SPS growth. Running carbon, performing regular water changes, and maintaining spacing between coral groups mitigates the chemical interference.

How do I know if my coral is happy? Full polyp extension, vibrant colour, visible growth over weeks, and feeding response all indicate a healthy coral. Retracted polyps, colour loss, mucus production, and tissue recession indicate stress.

When should I start adding corals to a new tank? Wait until the tank has fully cycled (ammonia and nitrite at zero) and has been stable for at least 2 to 4 weeks after cycling. Start with hardy soft corals. Wait 6+ months for SPS.

How close together can I place corals? Leave at least 3 inches between non-aggressive species and at least 6 inches around species with sweeper tentacles (Euphyllia, galaxea). Account for growth. What looks well-spaced today may be touching in six months.

Do I need to dip new corals? Yes. Dipping in a reef-safe pest treatment before adding to the display removes flatworms, nudibranchs, and other hitchhikers. Dip every new coral, regardless of the source.

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