Hmm... you have a couple of issues going on here. Dropsy is a symptom & not a disease itself. It can be caused by an internal infection, parasites, a virus or poor water quality. Which your cause may be, I am not sure but obviously water quality is not one of them.
Is the guppy the only fish with an issue right now (fin rot)? Does anybody else have any symptoms of any kind? Strange poop, odd coloring, sores, spots, flashing, lack of appetite, skinniness, anything? If fin rot is the only issue you are facing right now, I would opt for healthy water along with aq salt & see how he does. If he does not improve/starts to get worse, then you can consider treating with meds. I would opt for Maracyn Plus or Maracyn 2 for a med. Hope this helps!