HOLE IN THE HEAD....if you dont know what that is then i'm sure that might be it. if it hasnt healed and it looks like it hasnt. it prolly wont. no real known cause for it but it most cases it is poor watr quality which i'm sure you dont have. unless i see it in person i cant really say but it does look like it. remebr any fish is prone to it...and it shouldnt effect the way the fish behave or eats. thats what i think but i cant bee wrong...reason saying its HITH is because it looks like it was starting to eat away his bar on his head...