I have had my red cap oranda goldfish for almost a year now. I keep him in a 10 gallon tank with a 30 gallon filter. He is the only fish in the aquarium currently. I live at college and I wasn't allowed to have an aquarium in the dorms, so I had to leave my fish at home. When I came home for break in January, I noticed that he had severe fin rot, so I bought a bottle of Melafix and treated the aquarium accordingly to its directions. My parents followed up with treatments while I was at school and when I came home for spring break in March, his fin rot seemed to have been cured. His fin had even grown back. However, I just came home for the summer last week and I noticed his fin rot had come back, but it is not as severe as it had been in January. This time, I bought Maracyn Two to treat it. I finished the week long treatment today and I'm planning on doing a water change tomorrow and giving the tank a good cleaning and then, I will treat him again for fin rot. I was wondering though, what is causing the fin rot. Both times I had noticed it, I took a water sample from my aquarium to Petsmart and both times, my water quality was fine. The temperature is 70 degrees, which is in the safe zone. I change my filter cartridge monthly and there is no excessive algae in the tank (only a small patch in the one corner). Any ideas on what could be the cause of the fin rot? Thanks for the help!