Hope this is not too late for your fish
Dropsy is a non specific term used to describe all odematous coonditions in fish
it can apply to both abdominal distension with fluid - ascites
and also skin odema, this often leads to the scales protruding in a pine cone fashion
eye's may also push forward with fluid accumlation behind them
Any condition that affects osmoregulation can cause dropsy
ie gill, kidney disease
statisically diagnosed disease's , and doont forget not many people bother seeing a vet or having a post mortem include kidneys in goldfish and abdominal neoplasia in older koi
antibiotic treatment is often aimed at kidney disease, lack of specific diagnosis is probably why we fail to treat this condition effectively
possibly try isolating the affected fish, adding a combination antibacterial products(melafix with whatever your potion maybe) and possibly salt at low dose's purely as support for gill osmoregulation
unless of course he's an old carp
andrew