I'm having a problem with my two upside down cats and my pleco. The dorsal and tail fins seem to be getting fin rot over and over again. I have treated with salt (no help), some rather nasty medicine in pill form(worked but rot came back) and a product called Melafix(this seems to work the best but the rot comes back after 2 weeks or so). The tank is a 55g with 4 gouramies, 2 dojo, a gold barb and 3pristella tetras. No other fish have been bothered by this problem (other than watching their friends suffer). The tank gets between 15-20%water changes on a regular basis. What else can I do, I hate medicating so much. Thanks for any help.