If you are setting up a reef tank, you will likely want to stock the tank with colorful marine fish, crustaceans such as crabs and shrimp, and various other invertebrates like sea urchins, sea cucumbers, and anemones. These odd creatures all add tremendous interest to your tank ecosystem. However, the central feature of any reef tank is the reef itself -- made of coral.

Coral is a invertebrate marine organism that lives in compact colonies. Individual corals are polyps; polyps cluster together to form coral heads, although a coral head is in reality a myriad of genetically identical polyps, each of which can be only a few millimeters in diameter. Over generations, these polyps secrete calcium carbonate to form a hard skeleton, which is what we think of when we visualize a coral reef.

Although coral polyps can catch small fish and minuscule organisms such as plankton with stinging tentacles, coral obtains most of its food via a photosynthetic alga called zooxanthella. The polyps absorb nutrients from the algae’s waste. As such, for the algae to thrive, coral requires sunlight and mostly grows in clear, shallow water, never deeper than 200 feet. (Some varieties of coral that do not require zooxanthella as a food provider can grow much deeper in the ocean.) Corals used to create reef tanks generally require zooxanthella for nutrition, and therefore adequate lighting is a primary consideration for any successful reef tank.

There are five broad categories of coral used in reef tanks. Large polyp stony coral, or LPS coral, are prevalent in reef tanks because of their hardiness and beauty. LPS coral readily creates a calcareous skeleton, and thus requires a minimum calcium level in your tank water; 400-450 ppm (parts per million) is ideal. Check your water chemistry often; insufficient levels of calcium can hinder skeleton production, but excessive amounts can “lock up” other essential minerals and elements in your water. Lighting for LPS coral should be medium to high range, 5-7 watts per gallon, with a mixture of actinic light (a “cool” blue light that promotes photosynthesis) and full-spectrum daylight bulbs (which will provide a more natural color as balance).

LPS coral includes brain, frogspawn, hammer, bubble, fox, and acanthastrea coral. Placement is very important; some LPS coral has sweeping tentacles that can sting other corals in the vicinity in the event of contact.

A second broad category of coral, zoanthids, are commonly referred to as button polyps or “zoos”; these are appropriate for beginning hobbyists, as zoos can flourish in a variety of conditions. They can range in color from deep brown to neon blue, adding a great visual element to your aquarium. Also, these corals spread rapidly, quickly filling empty spaces in your aquarium. Zoos require a steady current to capture food and absorb nutrients in the water; they primarily rely on photosynthesizing zooxanthellae for their sustenance, but also require supplemental food. Lighting should be moderate only; excessive lighting may be harmful.

Zoos are invasive and will grow onto nearly anything that is adjacent, including other corals and aquarium equipment. If you want to contain your zoos’ growth, isolate them with a ring of uninhabited sand substrate, as they are reluctant to spread directly onto a sand bed. If your zoos do invade another coral’s space, you can separate the two with a razor blade.

Mushroom corals, too, are good corals for beginners; mushrooms have soft bodies and are available in almost any imaginable color and size. They do not require intense light or strong water current, so if you’re using high-intensity lights for some other feature of your tank, keep mushroom corals near the bottom. Also, mushrooms do not produce a calcareous skeleton, so the calcium level in your tank does not have to be kept high. They do use photosynthesizing algae for nutrition, but will also eat solid food that may be present in the water.

Soft corals form a very broad general category including leather corals, carnation corals, chili corals, pipe organs, and Kenyan tree corals; each of these subcategories has special requirements, so it is hard to generalize. Pipe organs and leather corals rely on photosynthesizing algae for nutrients and also collect food via currents; thus, they require a strong flow. Leather coral in particular needs strong water flow because it occasionally sheds its outer skin, which must then be carried away by currents. Unlike most corals, pipe organs do not have sweeper tentacles, and thus are not a danger to other corals.

Carnation, chili, and Kenyan tree corals, on the other hand, do not rely on photosynthesizing algae, and thus require daily feeding. They should be positioned away from direct light, for instance under an overhang, and they require very strong water flow. These corals are difficult to keep and not recommended for beginners.

Finally, there are small polyp stony corals, or SPS corals. These polyps produce a strong calcareous skeleton; in the wild, they form the base of every major coral reef around the world. They rely on photosynthesizing algae, but these algae are present in reduced levels in SPS coral. Light must therefore be high intensity (7-10 watts per gallon, a drastic increase over lighting needs of other coral types), and the water must provide a constant supply of calcium. Because of the reduced levels of algae, it is often necessary to supplement SPS polyps’ diet with phytoplankton or a specific coral food such as “marine snow.”

Once you have decided what kind of coral you want, you can begin planning the rest of your reef environment. Depending on what life forms you are adding, certain creatures and plants should be added before others. Coral generally is added after other invertebrates (specifically invertebrates that help keep your tank clean); be sure that corals are attached to “live rock” at the bottom of your tank, and not merely laid on the sandy substrate. Live rock refers to rock (calcium carbonate) that contains many forms of micro- and macroscopic marine life. Your coral dealer will have a plentiful supply, and can provide specific instructions on installation.

With proper care, you can enjoy your marine reef tank for many years.