i have the same lungfish an that happened to the back end of my lung fish when my 3 plecs took a liking to his taste. safe to say they were promptly rehomed and he recovered but it took a rather long time what other fish are in the tank?i caught mine in the act when I came in one night an found them on him at night. if u dnt have sucking type fish then yes it is water parameters so a nice water change an a filter with gd uv will help.