Help asap!!! (balla shark)

[Sharklover]

I have had 2 balla sharks in my tank for about 5 months and they were perfectly healthy along with a few other fish in my 37g and now all of a sudden one of the balla sharks dorsal fin(fin on top) is all red in the middle but evry thing else on him and the rest of my fish and in my tank is fine does anybody know what it could be?ps-i dont have test kits for anything because the tank is simple and very stable and never had any probs in tank since ich out break in first week of them in tank but that was over 4 months ago,any clues to my problem would be appreciated and not tank size crittiks pls cuz in a few more months they will be out.

 

[alLexX]

im not sure whats wrong with the fin,but a balla shark gets waaaay too big for a 37 gallon aquarium,they can get up too a foot long.