i lost one with what looks similar, it was almost as though the fish was slowly rotting, i tried everything i could think off, i put it into a seperate tank with salt and an air stone, i can't remember what the treatment i put in with it was called, i tested my pond water and everything was normal, when it died and i got it out to bury it, it absolutely stunk of rotting fish, so i would also be interested in what others think it could be