Possible betta fin deterioration?

ive recently added two dwarf gouramis to my tank (i posted a question earlier but didnt get the results i was hoping for) with my betta, gillis, since they have moved his fins have gotten shorter on the bottom and he seems a lot more stressed out than usual. which i know is because of the gouramis (i only bought them out of anticipation of my new 55 gallon :) which gillis is going into promptly anyways my real question is his fins look frayed or bent like an L instead of straight and flowish and his tail fin is shorter than when i first got him i have had him on melafix for the last two weeks and just a few minutes ago i found another piece of fin on the side of the tank. whats going on? ( i seperated him from the gouramis by placing him in his cup again which is in the water so it stays warm)