I think it's a water problem. I had two fish when I moved into school, Roxas (male), Gypsy (female). When I put Gypsy back in her tank she died that next morning, then I bought another fish, that one died the nest day, then bought a another one and that one died in less then 6 hours. Cleaned by last first, Roxas' tank yesterday. Do what I always do and that never seemed to bother him, and he died. I have multiple fish tanks and my first two fish were just fine over the summer in their tanks. Roxas had fin rot the whole summer but pretty much recovered from that and he was in the same 2 gallon tank since april. I don't have cycled tanks because that's never been an issue, and I don't know how to do that.