At your local fish store it will say on the box if it will treat dropsy as far as medication goes I am not ontop of things in that area, what I might recommend wouldn't be what others would or would not recommend....(Most the medications I recommend really arn't for begginers as in Peracide B or A, a negative Ion of some sort etc sometimes some can be found in the marine section at your lfs) main thing is stick to one med when treating it and keep clean water, even daily water changes if you need to with a water conditioner to help the fish with the stress.
Something that is eaither solution or powder would be best as you could mix this with food, an anti biotic will say on the box this Nitrofurazone, you have to remember when treating dropsy that it is NOT a disease but a sign of another disease that caused dropsy! Problem with dropsy is it can kill fish before you even attempt to treat it and it really is a hard one for scientists. When treating it you have to keep in mind that you not only should be treating the water its in but also treat the fish insides. I found a site with good information on this maybe it be advise you read it to understand how it actually works
http
/www.nippyfish.net/dropsy.html
Where salt baths is recommended by members, in later stages its best to stay away from salt, the kidney on the way it works with the dropsy will absorb the salt but make it worse for the fish so the signs become stronger. Salt is a no no when treating dropsy in later stages in beggining stages not sure I have always herd people who have used salt baths with success and also with painful outcomes because dropsy is not a specific disease and the name dropsy speaks for many specimens you are not actually sure what you are up against
Dropsy I think is best suited to say its like cancer in humans, cancer spreads especially when not treated and leads to death and not all treatments work... same goes with dropsy
Edit
negative, what i meant by that was gram negative spectrum and a gram postitive sorry just reread the my post