Swim bladder disease
Swim bladder disease, also called swim bladder disorder or flipover, is a common ailment in aquarium fish. The swim bladder is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming. A fish with swim bladder disorder can float nose down tail up, or can float to the top or sink to the bottom of the aquarium. Causes. Fancy goldfish are among the fish most commonly affected by this disorder. The disease may be  caused by intestinal parasites or by constipation induced by high nitrate levels from over feeding. Remedies. A remedy, which can work within hours, perhaps by countering constipation, is to feed green pea to affected fish. Fish surgeons can also adjust the buoyancy of the fish by placing a stone in the swim bladder or performing a partial removal of the bladder.