Corals have endured all sorts of environmental conditions throughout the 500 million years they have been in existence. Keeping a live specimen with such a rich legacy demands some general knowledge of how they live in nature. Then you can begin to understand how certain behaviors may play out in your tank.

The awesome beauty of the coral reef has long been a popular draw for tourists and marine enthusiasts. But this attraction has not come without cost to the health of many of reefs. Over the last decade, scientists have tried to raise public concern for these environments as they have watched thick strands and mats of algae progressively overgrow the colorful coral reef ramparts. With so many species living in and around the reefs, what affects their health for better or worse is not an easy question to answer. Only recently have researchers even come close to agreeing on what may define a "healthy" reef.

The variously shaped and colored coral heads that you see on a reef are actually communities of many individual animals, called polyps, which themselves are, on average, no bigger than an eraser-head. Take a close look at a polyp and you'll see tentacles that reach for the sky from a trunk-like base, around which the polyp has built a calcium carbonate home, or corallite.

How Corals Changed the World

One would never guess that this tiny creature has reshaped planet Earth more than any other – including humans. The hermatypic corals – the reef-builders – form large colonies that can span thousands of miles, such as the Great Barrier Reef of Australia. Only a thin veneer on the surface is actually alive because as a polyp grows, it builds its new home on top of the old one.

Over geologic time, this upward and outward growth has created kilometer-thick slabs of pure limestone, some of which have been folded into mountain ranges by tectonic forces. In some cases these thick layers of limestone reach thousands of feet into the Earth as they do under parts of Texas and the Persian Gulf, where the empty corallites have become natural repositories of our oil deposits.

While living, reefs provide natural protection to coastal areas, nursery grounds for several important fisheries – and plenty of tourist dollars. And recently, scientists have discovered that many reef species may hold the key to human ailments, such as cancer and arthritis.

Maintaining Water Quality

Although reef invertebrates are very different in form and function, the one thing they all have in common is a need for water quality. Keeping a stable environment is the greatest challenge to keeping a healthy reef system. Clear water that is free of pollutants, organics and sediment, and a hard substrate on which the larvae can settle, are absolute musts to keep corals happy.

Corals, animals of the class Anthezoa in the phylum Cnidaria, are most prolific in the tropics, between 25º N and 25º S latitudes, because water temperatures there rarely get below the mid-60s or 70s – the minimum range of most hermatypic corals. They prefer salinities of about 35-38 ppm, although some of the most spectacular reefs in the world, such as those in the Red Sea, live in an average salinity of 40 ppm.

Because they have small photosynthetic symbionts living in their tissues, corals need at least 10 to 12 hours of light every day. The symbiotic algae, called zooxanthellae, not only give the corals their color but also provide necessary nutrients to their host that in turn allow the coral to build and maintain its little limestone apartment.

When corals are stressed, such as when water temperatures or salinity get too extreme over an extended period of time, they will eject their zooxanthellae, becoming pale and faded. If poor conditions continue, corals will eventually turn white. This phenomenon, known as "bleaching," was observed on a grand scale around the Galapagos Islands during the 1998-1999 El Niño event. Your tank corals will do the same if they aren't feeling well. Sometimes, coral can recover, sometimes it cannot.

Preserving the Reefs

As currents weave through the reef tract, the polyps graze on larvae and plankton, which they pluck from the water with arms laden with thousands of stinging cells. Under conditions that are too high in nutrients, called "eutrophic" in natural systems, algae will overgrow the coral polyps in a blink, depriving the coral of food and its symbionts of essential light. Fertilizer runoff and sewage are the main contributors to such a transformation.

Because reef environments are among the most productive in the world, providing a nursery for, and sheltering, many different (and delicious) species, such as lobster, grouper, snapper, and eel, they experience tremendous fishing pressure – and the destructive practices that often come with it. The use of explosives, poisons and careless netting (which damage the mucous coating on a coral community) are common fishing methods. As a result, many reef areas have become little more than rubble mounds. In Southeast Asia, for example, local fishermen now travel thousands of miles to fish along distant reefs in the South Pacific in an effort to satisfy the hungry sushi market.

Reef species are also heavily fished for in the hobby aquarium trade, which you as a conscientious reef-keeper must remember. As an aquarist wanting to keep corals, which are usually harvested from the wild, you cannot deny that you are part of the problem. But if you keep your coral healthy, and keep only those aquarium species that are bred in captivity, you may instead become part of the solution.