Pick up some Maracyn & Maracyn II at your lfs, and dose according to the directions. I would use more epsom salt than 1/8 teaspoon per 5 gallons, I start with a teaspoon per 10 gallons, increasing by a teaspoon per 10 gallons for a couple of days for a maximum of three teaspoons per 10 gallons.
Dropsy is a symptom, not an ilness in itself, it usually indicates an internal bacterial problem. The antibiotics will help with the bacterial infection, the epsom helps to draw out fluid. It is difficult to treat, I had around a 50% success rate with platys some time ago.