Fishes can be divided into 3 main feeding categories: (1) carnivores, or those that eat only animal material; (2) herbivores, or those that eat only plant material; and (3) omnivores, or those that eat both plant and animal material. The herbivores are specialists in feeding on the marine plants or algae. Since the algae that grows on the glass and decoration of the aquarium is not in sufficient quantities to satisfy the requirements of your fish, additional food must be found. Frozen lettuce, spinach, and other green vegetables are often accepted as substitutes. Wash these well before adding them to the aquarium. Dried foods with chlorophyll content and the fresh-water "Molly" foods are sometimes accepted. If you wish to feed natural marine plants to your herbivores, disinfect them first as they can carry unwanted "bugs" which can raise havoc with your prize specimens. Herbivores, usually out of hunger, will eat animal material. This does not necessarily mean that hey will thrive. They are missing something when they do not get their proper diet and may eventually die for lack of it.
Carnivores are easier to feed. Brine shrimp are readily available in several forms. For smaller specimens brine shrimp eggs can be hatched and the baby brine shrimp can be used as a staple in their diet. There are several popular varieties of brine shrimp today. The San Francisco Brine shrimp is smaller when hatched than the Salt Lake (Utah Brine Shrimp) species. Both are hatched in a brine solution and full instructions come with every pack. Brine shrimp need food and will eventually starve to death if not eaten quickly by the fishes. After a day or two the nutritional value of the brine shrimp drops very low so they should be replaced by newer ones. Try to keep the egg shells out of the aquarium, they can foul the tank, or, if eaten, cause digestive problems. Newly hatched brine shrimp comes in frozen packages.
For larger food brine shrimp adults raised by the aquarist can be used.
Freeze-dried brine shrimp have an advantage in that they can be kept wihtout refrigeration. They also have less of a tendency to foul the water and they are sterilized.
Naturally, the diet of your fish should varied whenever possible. Some of the foods fed to fresh-water fishes will do very well for marine fishes. Freeze-dried tubifex worms, Daphnia, enchytrae, white worms, and mosquito larvae are all acceptable.
Of the strictly marine foods chopped clams, mussels, crabs, shrimp, or fish all are highly desirable. Frozen or freeze-dried prawns, shrimp, crabs, brine shrimp, or other crustaceans, as well as tubifex worms supply a ready substitute. Dry foods like flake foods, crab meal, and shrimp meal are also excellent.
Omnivores will accept most of the foods mentioned above. They can usually exist on a pure animal diet but do somewhat better if plant material is also fed. Freeze-dried Tubifex with Chlorella algae is ideal for omnivores.
Many fishes when hungry enough will eat whatever is offered. Sometimes, however, certain species are reluctant to start feeding in new and strange environment. If other fishes are present which are good feeders, the newcomers will quickly imitate them and take their place at the "dinner table."
The type and frequency of feeding naturally depends entirely on the kinds and sizes of fishes in your aquarium. Larger fishes (over 1.5 inches) in general can be fed once per day. For smaller sized fishes at least twice a day feeding (more if possible) are recommended. For certain species, those that eat very little but often (butterflyfishes), a continuous supply of food, probably brine shrimp which can live in the tank with the fishes, must be provided. Brine shrimp is phototrophic, that is, they are attracted to light, and will accumulate where the light intensity is stronger. If brine shrimp are wanted in the lower levels of the aquarium place a small light at the desired level.
Feed sparingly and you will have your fish for a long time; overfeed, and you may lose them overnight. Your fishes are seriously harmed when the water becomes foul due to the presence of decaying, uneaten food, the source of billions of harmful bacteria. When the water becomes foul in a freshwater tank it can be easily replaced. Marine water on the other hand is expensive to replace. A little care when feeding can spare a big headache later on.