Popeye Treatment

If using medication, then something for bacterial infection (such as eSHa 2000 or Waterlife Myxazin) is best.

Edited to add: If torn over decision on whether or not to medicate, then I use API Melafix - means I don't have to take fish out to hospitalise them for medication (or worry out about adverse consequences on invertebrates ie my nerite snails & having to temporarily rehome them).