I had to take my Clarkii clown out of the main tank because he wouldn't stop harassing and biting the fins of my new yellow tang. Recently he developed a terrible sore on his mouth, and it appears to affect the inside and outside. He can't seem to totally close his mouth, and he is not eating. The tank had not seen a fish in at least 6 months, so I think I can rule out any parasite that needs a host. I'm leaning towards a bacterial infection, but my wife thought it looked sort of like a tumor. Any suggestions?