Sorry to hear about your betta, Keeper
Are there other fish besides the pleco in the tank with him now? He could be stressed over tankmates. How did you acclimate him to the new tank? Are the temperatures and pH the same? A large change in these parameters could cause stress.
The others are right about the MelaFix/BettaFix. It can have an adverse effect on the labyrinth, and there are better meds for bettas.
I would watch the fins for now -- does the edge look like it's smoothing over and healing already? If it is, just do some extra water changes -- he'll heal better in nice, clean water. If the fins are black/stringy, then the betta may need an antibiotic. I have used Jungle Fungus Eliminator, TripleSulfa, Kanacyn, and Maracyn 1 or 2 sucessfully on bettas.
What is the temperature of the tank? Bettas prefer 80 degrees. I had, at one time, a betta and a pitbull pleco together. This was only a temporary setup. While they were together, the temperature was 76. By the time I moved the betta, he had some finrot. I thought it was because the temp was too cool for him. Now the betta and pleco are in separate tanks. The pleco tank is at 76 degrees and the betta tank is at 80 degrees, and the betta has healed nicely.