Feeding

Like most fish, goldfish are opportunistic feeders. When an excess of food is offered, they will produce more waste and feces, partly due to incomplete digestion of protein. Overfed fish can sometimes be recognized by feces trailing from their cloaca. Goldfish need only be fed as much food as they can consume in one to two minutes, and no more than three times a day. Contrary to some common belief, extreme overfeeding will not increase the size of the fish but can actually be fatal, typically by bursting of the intestines. This happens most often with selectively bred goldfish, which have a convoluted intestinal tract as opposed to a straight one in common goldfish. Novice fishkeepers who have newly purchased ryukin, fantail, oranda, lionhead or other fancy goldfish will need to watch their fish carefully for a few days, as it is important to know how much the goldfish will eat in a couple minutes of time.

Goldfish food has a lower protein and higher carbohydrate content than conventional fish food. It is sold in two consistencies - flakes that float at the top of the aquarium, and pellets that sink slowly to the bottom.

Goldfish enthusiasts will supplement this diet with shelled peas (with outer skins removed), blanched green leafy vegetables, and live food such as daphnia and bloodworms. Young goldfish also benefit from the addition of brine shrimp to their diet. As with all animals, individual goldfish will display varied food preferences. In any case, it is better to feed them a variety of foods listed above.

From: http://en.wikipedia.org/wiki/Goldfish