Dropsy is a symptom, not itself a disease. It indicates 1) kidney failure or toxicty from poisoning (untreatable) 2) liver failure, same causes, plus improper diet (also untreatale by the time it is symptomatic) 3) toxicity from bacterial septicemia ( may or may not be treatable, depending on the sensitivity of the bacteria involved and whether or not the appropriate antibiotic can be found and the dosage delivered into the fish's body).
The net result is that by the time the symptom shows, treatment rarely works. When several entirely different fish in a tank develop the same symptom over time, the commonest cause is an antibiotic-resistant bacterial septicemia.