Corals

The single animal is a polyp: a soft body with one opening, the mouth, ringed by tentacles. The tentacles carry stinging cells that catch food. Most corals are colonies. One polyp buds into two, and those into more, and they stay joined by a sheet of tissue that lets them share food.

In a stony coral each polyp sits in a cup in the skeleton and can pull back into it. The living coral is only a thin layer of tissue over the skeleton it has built. Damage to that tissue is damage to the animal, and bare skeleton is no longer coral.

Stony corals build a skeleton of calcium carbonate, taking calcium and alkalinity out of the water to do it (Module 19). They're the corals that build reefs. As they grow they use more, so a system full of growing stony corals needs steady supplementation.

Soft corals have no hard skeleton. Their bodies are flexible, stiffened by tiny hard pieces inside the tissue, and their polyps have eight feathery tentacles. Many soft corals release chemicals into the water that irritate other corals (Module 24).

Acropora and Montipora are the best known. Their polyps are only a millimeter or so across, over thin tissue, and they grow as branches, plates or crusts. They're fed by broadcast (Module 26).

They do best in strong, turbulent flow from all directions, never a direct jet, and at about 200 to 350 PPFD, higher for some (Module 18). A new one starts low in the tank or on a rack for the first few weeks and is moved up slowly.

The key is stability, and alkalinity is the number that matters most. When it swings, these corals pull their polyps in, lose color, and lose tissue. This course's range is 6 to 10 dKH, with 7.5 to 8.5 preferred (Module 6); for these corals, holding it steady matters more than where in that range it sits. High nitrate and phosphate turn them brown and stall their growth. A new system swings, so these corals go into a mature one (Module 9). Montipora is easier than Acropora.

Hammer, torch, brain and chalice corals are in this group. The polyps are large enough to hold a piece of food, so they're target fed (Module 26). They're still stony corals: they take calcium and alkalinity from the water to build skeleton (Module 19).

Most do well in low to moderate flow and at about 75 to 150 PPFD, though light needs vary a lot by species. In a moderate current they inflate fully; in too much they stay closed, and a direct jet tears the flesh. Many reach out with long stinging tentacles, so they need more room than their size suggests (Module 24).

Leather corals and mushrooms are the best known. They generally do well in low to moderate light, about 75 to 150 PPFD, and low to moderate flow. They don't take up calcium and alkalinity the way stony corals do, and they tolerate small swings in the water better.

Hardy doesn't mean harmless. Many soft corals grow fast and release chemicals that irritate other corals, which carbon helps remove (Module 24). A leather coral that closes for days and sheds a waxy coat is doing something normal: it reopens afterward.

Zoanthids and the larger palythoa, often called zoas and palys, are kept alongside the soft corals. A colony is a mat of tissue that spreads over rock, covered in flower-shaped polyps. They're hardy and fast-growing, they don't take up calcium and alkalinity the way stony corals do, and they tolerate a wide range of light: most keepers aim for about 75 to 150 PPFD. Low to medium flow suits them, enough to keep debris from settling on the mat. A spreading mat can overgrow its neighbors, and neighbors can sting it.

Some zoanthids carry palytoxin, a very strong toxin (Module 2). There's no way to tell which by looking, so all of them are handled as if they do: gloves and eye protection, under water where possible, and no bare hands with cuts. Rock or plugs with polyps on them are never scrubbed, boiled or put in hot water, which can put the toxin into the air.

Most corals carry algae in their tissue that turn light into energy (Modules 3 and 18). More light isn't better. Photosynthesis rises with light, levels off, and then falls, and most corals reach that level at moderate light. Past it, extra light adds no growth and can do harm. Brighter light can bring out more color, at some cost to growth. Corals also catch food, which gives them the protein that new tissue is built from (Module 26).

Reef systems are normally run at 26 to 27 C (Module 14). Corals do best in stable water: a steady number that isn't ideal does less harm than a number that swings (Module 6). After a coral has settled, any change to its light or flow is made over weeks, not days.

Findings for one coral don't always hold for another. In trials on one flowerpot coral (Goniopora), it grew and opened best under blue and violet light and lost algae with no light, and it survived salinities from 25 to 40 ppt but grew and opened best at 30 to 35 ppt.

Non-photosynthetic corals have no algae in their tissue, so light gives them nothing, and all their energy comes from food. Sun corals are the best known. On a reef they live under overhangs and in caves. In a system they're placed where they can be reached, because they're target fed meaty food several times a week, and some need it every day.

A sun coral that's closed can be coaxed open: a small amount of food near the polyps brings them out within minutes, and then it's fed. An underfed colony starves slowly, opening less and less each week. Feeding this heavily adds a lot of waste, so the system needs strong skimming and regular water changes (Module 17). These are demanding animals. Know what a coral eats before it's bought.

Flow brings a coral its food and carries its waste away (Module 13). With too little, detritus settles on the tissue, the polyps stay closed, and the tissue underneath can die. With too much from one direction, a jet of water tears the tissue, most easily on fleshy corals. Flow should be turbulent and indirect, not a pump aimed at a coral.

The four groups give a starting point for light and flow. Place a coral for what its species needs, starting low and moving it up over about two weeks (Module 18). Leave room. Corals grow toward each other, branching and plating corals shade what's below them, and many corals sting any neighbor within reach (Module 24).

A healthy coral puts its polyps out at its usual times, with full tissue and steady color. Many corals extend furthest at night to feed, so learn what's normal for each one (Module 3). Polyps closed for an hour mean little. Polyps closed for days mean something is irritating the coral.

A coral turning pale or white with its tissue still in place is bleaching: it has lost its algae and is starving, and it can recover if the cause is fixed. A coral turning brown has more algae than usual, from too little light or high nutrients. Bare white skeleton spreading from the base or the tips means tissue is dying. Fast tissue loss is an emergency (Module 30).

Work through what changed. Water: a swing in alkalinity, salinity or temperature can close a coral even when the final number looks fine. Light: a new fixture, a coral moved up, or a neighbor that has grown over it. Flow: a pump moved, clogged or failed. Neighbors: something new, or something that has grown within reach. Nutrients: too high, or driven to zero, where a coral goes pale and hungry (Module 17).

More than one of these is often true at once. Check them all, then fix each cause, bringing any number back slowly. Leave the coral where it's unless its place is the problem: moving it again and again is one more change. Pests are covered in Module 31.

A colony grows by budding new polyps, all copies of the first. A broken piece with living tissue can attach and grow into a new colony, which is how corals are propagated (Module 34). Corals also reproduce by spawning: many release eggs and sperm into the water, and some release larvae that are already formed.

In a closed system a spawn clouds the water and sends the skimmer foaming over. What was released rots if it stays, so it's taken out: fresh carbon, fine mechanical filtration changed as it clogs, and water changes, with ammonia watched until the water clears (Modules 15 and 17). Cloudy water with an anemone deflated or clams looking off, while everything else is fine, points to a spawning event.

Start with the coral. Where on it's the damage: all over, at the base, on one side? Damage on one side points to something on that side, a pump or a neighbor. A whole colony gone pale or brown points to light or nutrients. Skeleton showing at the base points to the water.

Then check everything, because coral problems are often a combination: the light was raised too fast and the alkalinity swung; the flow dropped and the nutrients hit zero. Fix each cause, slowly.