My poor fish is struggling with fin rot. He had it a few weeks ago, so I put him in a smaller tank with 1tsp AQ salt per gallon and did a water change every day. After a week, he started looking a lot better, so I put him back in his regular tank. Two weeks later, and the fin rot is back! I don't know what I'm doing wrong; I changed the water in his larger tank every other day (50% on the first day, 100% on the second), and the parameters are fine.
He's back in his smaller tank again so he can heal up, but I know that I'm not supposed to treat him with AQ salt for more than 10 days. After he starts looking better, should I keep him in the smaller tank (without AQ salt) to make sure he is all better?
What factors--besides poor water conditions--can cause fin rot? There is a filter in his tank, but I put filter sponge around it to slow the current. Could the filter still be the problem? There are no sharp plants/decorations in his tank that could have scratched his fins either.