I have four silver dollars and one angelfish in a 55 gallon tank. I change 35%-50% of the water weekly. For a couple months now, I've noticed two of the silver dollars and the angelfish have fin rot. I treated the tank with a large bottle of melafix. That didn't help, so I treated the tank with maracyn. That hasn't helped either. The fin rot has progressed slowly. The two silver dollars have lost half of their tales and the angelfish has lost membrane between the rays of it's dorsal fin. The other two silver dollars show absolutely no sign of fin rot. I don't know what is causing the fin rot or how to stop it. Any suggestions? Thanks.