Dropsy is a common disease among fresh-water aquarium fish. The name is from an old name for Edema in humans. Dropsy is caused from a bacterial infection of the kidneys, causing fluid accumulation or renal failure. The fluids in the body build up and cause the fish to bloat up and the scales to protrude. You can best see this by viewing your fish from the top. Fish may also stop feeding, appear off-colour, become listless and/or lethargic, have sunken eyes, and hang at the top or stay at the bottom of the aquarium.

It appears to only cause trouble in weakened fish and possibly from unkempt aquarium conditions. Dropsy is not very contagious; however, if a fish is diagnosed with dropsy, it is important to remove it from the aquarium for treatment.An effective treatment is to add an antibiotic to the food. With flake food, use about 1% of antibiotic and carefully mix it in. If you keep the fish hungry they should eagerly eat the mixture before the antibiotic dissipates. Antibiotics usually come in 250 mg capsules. If added to 25 grams of flake food, one capsule should be enough to treat dozens of fish. A good antibiotic is chloromycetin (chloramphenicol). Or use tetracycline. Another approach is to raise the aquarium's temperature a few degrees - slightly higher than usual. Adding Epsom salts (Magnesium sulfate) to the water at (a rate of 20 mg/L) helps to encourage the fish to expel unnecessary damaging fluids.

Prognosis of fish dropsy is not good. By the time fish has swollen up enough that the scales begin to raise, the internal damage may be too extensive to repair. Most cases of dropsy are fatal.

Maintaining water quality is always extremely important. It should always be checked first, because it is often the cause of disease in aquarium fish. Frequent water changes can work to prevent the spread of disease by "watering down" the concentration of disease agents, and by reducing stress on the tank occupants.