Until fairly recently, keeping corals in a closed saltwater system (aquarium) was considered next to impossible. As the water quality science improved and hobbyists learned how to keep toxins (primarily nitrates and phosphates) that corals are sensitive to under control, and essential trace minerals at their proper levels, more corals were being successfully kept in aquariums. As coral lighting requirements became better understood and more powerful lighting systems were developed, corals are now being kept successfully. Understanding and simulating the varied water currents that corals require improved the success rate even more.
The final barrier to being able to keep any coral in an aquarium has come down to the ability to provide the specific food requirements for individual corals. Some corals' requirements are so specific to their environment in the wild that they are almost impossible to simulate in an aquarium. On the other hand, many corals seem to thrive in an aquarium without a specific feeding regimen, obtaining their nutrition from tank lights and the available food contained in the tank water.
Corals feed in one or more of 3 different ways.
This is the stuff that gives corals their vivid colors. It provides food to the coral from light via photosynthesis. This is beneficial because individual coral species light requirements can vary widely, even within a coral genus. However, this may provide only a portion of total nutritional needs.
If your coral has large tentacles and a visible mouth (i.e. many LPS Corals), it probably eats macroscopic or larger prey. Lobophyllia, Open Brain, Elegance, and Plate Corals are examples of corals that do best on larger food.
It's important to offer a variety of foods to find one or more that your coral will accept. This can include diced small fish, thawed frozen plankton, phytoplankton, krill, pieces of shrimp, squid, or clams. These are also known as octopus foods and many saltwater aquarists believe this simplifies coral feeding. DIY Coral Foods can be prepared for specific coral needs.
Unacceptable foods will be sloughed from the disk or not captured at all. Many corals require medium to strong currents to flush their surfaces of excess food particles. When using this method don't overdo it, as overfeeding can be a major cause of nitrate build up in a tank.
Indirect feeding happens when corals absorb Dissolved Organic Compounds directly from the water. Waste products and uneaten food are major sources of coral food. Many of the small bits of food, which the fish do not eat, are consumed by corals. This includes bacteria contained in the plankton for small polyped corals.
Research Your Chosen Corals
The particle size of food also determines what a coral can eat. A chunk of food the size of your little fingernail can be consumed by many Large Polyps Stony corals but will be of no use to a Zooanthid Coral. On the other hand, a Button Polyp can absorb Dissolved Organic Compounds directly from the water, as can many LPS and SPS corals.
In the end, researching the habitat of a particular coral in the wild will go a long way in determining what foods that coral prefers. Talking with other aquarists (i.e. Forums), online research and reading books and articles on specific coral species food requirements are excellent places to find information before purchasing a coral for your tank.