To one extent or another, corals do require a certain amount of light to survive. Some corals, however, depend less on light than they do on nutrients extracted from the water for their nutrition.
Types of Coral Food
While many species of corals contain symbiont zooxanthellae algae that produce nutrients by photosynthesis from light, most soft corals, zoanthids, and gorgonians depend almost exclusively on ingesting phytoplankton (small water-borne plants or algae) for their nutritional needs, as well as floating detritus and slow-moving invertebrate larvae.
Another important source of food for corals is bacterioplankton, which consists of free-living bacteria and the bacteria associated with decaying materials in the water (on mucus, dead plants, and other particulate matter) that are commonly called detritus or reef snow. Almost all corals feed heavily on bacterioplankton. Other food sources include floating eggs, zooplankton (free-swimming microorganisms) and non-swimming organisms known as pseudoplankton.
The fourth category of food utilized by corals is Dissolved Organic Material (DOM), which is absorbed across cell membranes directly into the coral from the water.
Many of the corals with larger polyps (e.g., Cynarina and Catalaphyllia) are capable of capturing and eating larger food items, including the occasional small fish. Some corals (particularly Gorgonians and soft corals) may select their food based more on the size of the plankton, rather than its composition. Some of the small polyp corals are also aggressive feeders, trapping and eating large food items.
If you have live corals in your aquarium, you are probably wondering what foods your corals eat to supplement the nutrition provided by their resident zooxanthellae algae. You could just make a slurry of a variety of different foods that cover the entire spectrum (the "shotgun method" approach) and load it into your aquarium, allowing the corals to select what they want from the mix. But, the uneaten food in the mix is guaranteed to increase your nitrate level in a short period. Or you can fine-tune the supplement to the requirements of your specific corals and target feed them with a turkey baster or syringe that applies the food directly onto the coral's tentacles.
Many corals will benefit from the food that you feed the fish and invertebrates in your tank. When meaty foods float by or land on corals, they will be consumed if the food is digestible by the coral. Copepods, Amphipods, Brine Shrimp and Mysis Shrimp will also be consumed by many corals. Copepods and Amphipods are quite easy to cultivate in a refugium. Brine Shrimp eggs can be inexpensively hatched and grown in a simple DIY Brine Shrimp Hatchery. It is difficult to generalize the food requirements for groups of corals (LPS, SPS or soft) as there are always a few renegades in each group that have a more selective diet. We highly recommend obtaining a good reference book on corals to determine what your specific corals feed on. One book that we highly recommend is Aquarium Corals - Selection, Husbandry and Natural History by Eric H. Borneman. The sections devoted to each coral provide detailed information on what the corals feed on in the wild. There is also an excellent chapter on how to feed corals in your aquarium.