Four months ago I got a delta tail male betta fish. He was housed in a 2.5 g tank with a filter and a heater. About two weeks later I noticed that his fins have tears in them, and the edges looked rusty and reddish, so I concluded he has a fin rot. I isolated him in a 1 g hospital tank without heater (the temperature in the room is around 76 F any was) or filter, but I changed his water every day. I treated him initially with aquarium salt and melafix, but when that didn't help I tried tetracycline, and a few weeks later kanaplex, which finally seemed to help. His fins seemed to get better, the reddish parts disappeared and I could see some new growth. After some time, since he looked quite unhappy and depressed in a hospital tank, I decided to return him back to his original tank that was cycled and had readings of 0 ammonia, 0 initiates, 0 nitrates. Two days later I noticed new tears and pinholes in his fins, so I returned him back to the hospital tank and started the treatment with aquarium salt again. When things got better again I bought a new 3 gallon tank for him, since the old one is now occupied by two ADFs. It is cycled (ammonia, nitrite and nitrate readings are 0), has heater and filter. Within two days of him being in the new tank his tail and even dorsal fin got totally ragged, like they are melting away and the edges look bloody. Otherwise, he seems pretty happy, he built a pyramid of a nest in the tank corner, he eats like a pig, swims happily around and since I got him he almost doubled in size. I am totally out of my depth here and don't know what to do anymore. I wanted to give him a nice home where he could be happy, but the only place where he seems to do ok is a betta bowl. The only thing that I didn't mention so far is that one of his pectoral fins was curling and twisted already when I got him, so I guess he might have some genetic flaw or predisposition for fin issues. Another thing that might be the cause of his fin problems is his behaviour - I noticed that he likes to get in to tight places and crawl between the things like plants, filter and tank wall, heater wire and the tank wall. Did anyone experience something similar? I would really appreciate if someone could give me some advice on how to proceed here.