So i have two blackmoors and recently one of them, who has always been the fatter one, is having trouble swimming. If he stops swimming he floats to the top and just hangs upside down. I know hes still alive cuz I see him breathing and if i move or tap the tank he swims around again. Is he sick or dying? I dont really know much about fish but I know his behavior is not normal. His belly has gotten bigger and he has a white stringy like thing coming from his butt. Someone have any advice for me? Please help.

Nossie covered it It is probably swim bladder disease which could be caused by a poor diet and/or bad water quality. Feel lucky you are being given prober advice, when I was much younger and less experience I had a fantail with a slight swim bladder problem, I was told to stop feeding it flakes and instead feed him sinking pellets. That was correct advice but I also needed to know much more! If I did the fish could have still been alive today