Dropsy does not consist of a single disease. Instead, various diseases and injuries cause dropsy. In dropsy, fluid builds up within fish, and they swell up. Depending on the cause, the symptoms and even the species of fish the prognosis and treatment may vary considerably.
A wide variety of problems can cause dropsy. Poor water chemistry, including high ammonia and nitrites or the wrong pH for your specific fish can trigger dropsy. For example, if you have an African cichlid that prefers a high pH and you keep it in water with a low pH, it may develop dropsy. Additionally, physical damage to the swim bladder can lead to dropsy. Infections, causing damage to various internal organs may also cause dropsy.
Whatever the cause, the main mechanism of symptoms and disease in dropsy is the swelling of the fish. This can stem from a specific organ swelling up. The swim bladder can swell up like this when something goes wrong. However, fluid building up inside the fish can also cause dropsy. Failing kidneys can lead to a buildup of fluid inside of the fish.
The main symptom of dropsy consists of the fish visibly swelling up. If they have large scales, this may create a kind of "pinecone" look to the fish. Sometimes, the fluid buildup may cause the eyes to distend, a condition called popeye. Fish with dropsy may also have trouble swimming, either from damage to the swim bladder, pressure on the swim bladder from fluid in the abdomen or changes in the fish's buoyancy due to the built-up fluid. Since dropsy is a symptom, not a disease, other symptoms will often present themselves at the same time.
Unfortunately, in most cases dropsy is the last sign of irreversible damage. However, you can sometimes save the fish. If you test your water and find that your water chemistry is inappropriate for your fish, you can perform a water change to remove nitrite and alter the pH -- slowly. Sudden changes in pH can hurt a fish even more than the wrong pH. You can also try antibiotics if you have reason to believe an infection is at work. If a fish does not respond to treatment, doesn't improve with better water conditions and is clearly suffering, you may want to consider euthanasia. Fortunately, dropsy is usually not contagious; even the cases caused by infection seldom spread to other fish.