If its fin rot. Pristine water is the key. Lots of wc.
from my experience, water changes are the best options.
i know there's a lot of misconception around melafix, but if the fin rot gets worse and is persistent, it can work great. the only real link between melafix and problems associated with it come from dosage error. so i'd even underdose just to be sure.
Would there be a problem with treating the entire tank with melafix, as opposed to hospitalizing the Guppy, if it comes to that.