Dropsy is not a disease itself but a symptom of another disease. The other disease could be anything - bacterial, viral or even parasitic. The fish swells up and it's scales stick out when the underlying disease has damaged the fish's kidneys so it retains fluid. Usually by the time this happens, so much damage has been done that the fish will never recover.
The probelm comes in trying to identify the underlying disease. There are treatments for bacterial and parasitrc infections, but not viral infections. But we need to know what's causing the infection to know the appropriate medication.
As for feeding, my fish are fed once a day, and not as much as you'd think they should be fed. If any uneaten food reaches the bottom of the tank, feed less food.