The cause of dropsy is an inability for the kidneys to expel water from the fish. The causes are kidney failure which antibiotics tend to exacerbate due to the antibiotics hammering the kidneys, or a large open ulcer, usually on the bottom of the fish, which is a breach in the membrane called skin, allowing pond water to flow freely into the fishes tissues trying to equalize the salinity of the pond with the salinity of the fishes blood, about 0.9%. It is that breach in the skin that can effectively be corrected by getting the pond salinity at or near the 0.9%, removing the majority of the water movement, allowing the kidneys to catch up. Treatment of the open ulcer is best done with antibiotics as stated by Marilyn, injectibles being the most effective, but Tricide Neo can be used in the event you are unable to source injectibles. Injectible antibiotics must be filled by a veterinarian and they are reluctant to prescribe a treatment for a fish, unless they are an aquatics vet, or you have an extremely good relationship.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard