Goldfish swim upside down when they have swim bladder disease. Goldfish and other species of fish may also swim sideways or do not have the ability to swim to the bottom of the tank if they have this disease.
Also called "swim bladder disorder," "floating disorder," and "SBD," this disease typically occurs when a fish's swim bladder has be compressed due to overeating, rapidly eating or gulping air. A fish's environment can also cause swim bladder disease. If the water temperature in a fish tank is too low, the digestive process slows down and can cause the intestines to become enlarged. The enlarged intestines then compress the fish's swim bladder. Less common causes of swim bladder disease includes kidney cysts, egg binding, fatty deposits in the liver, bacterial infections, parasites and other enlarged organs that put pressure on the swim bladder.
Swim bladder disorder is treatable. Fish owners should make sure the water temperature in the tank is at or close to 80 F, and they should lower the water level to make it easier for the fish to reach the surface. They should not feed the fish for three days. Then, the owner removes the skin from a cooked pea to feed to the fish. Owners can feed their fish a pea instead of flakes until it is swimming normally. If needed, owners can hand-feed fish and provide broad spectrum antibiotics.