A fish displaying bloat, especially a distended abdomen, most likely suffers from swim bladder disorder. A gas-filled sac located toward a fish's tail allows the fish to float. However, the swim bladder may become compromised because of constipation, overfeeding, bacterial infection or trauma, causing the fish to bloat. Other symptoms include floating on its back or side or inability to dive.
Reduce the quantity of food you are feeding your fish. Overfeeding often causes bloat and swim bladder problems in pet fish. Though the amount of food required varies greatly depending on the size of the fish, kind of fish and type of food it is fed, it is generally best to feed fish small amounts a few times daily rather than one or two large feedings. Feed the fish only as much as it will consume within a few minutes.
Feed your fish one cooked pea daily. Peas help break down impactions in the fish's gut.
Test the pH, nitrates and nitrites of your tank frequently using a water test kit. Poor water quality causes bacterial infections which inflame the swim bladder. Perform water changes or add chemicals to correct the water parameters as necessary.
Quarantine and treat your afflicted fish with erythromycin, tetracycline, minocycline or other fish-safe antibiotic to clear up any bacterial infection.
You can fast your fish for one to two days without worry. Only a fish-knowledgeable veterinarian can positively identify the cause of bloating by testing the inside of the swim bladder.