I'm reasonably sure it's not fin rot but since he's never had it I'm not really familiar with it. It's ONLY his tail that's affected and he's missing a big piece in one spot and smaller pieces just above it. The area is the top of his tail. There aren't any holes and his dorsal and lower fins are fine. So, since my water params are fine (0,0,10) should I just keep an eye on him and wait for it to heal? I'd posted that he was letting the filter suck him in and he's stopped that but I'm wondering if that's what was responsible.