Green wolf eel eating himself?

[Kenmx10]

I have a green wolf eel that appears to be eating his tail. I thought it must have been some kind of tail rot so I treated the tank with ammoxicillin 250mg for a week. Still not getting better. You can actually see his mouth marks were he's been eating at his own tail. I started to feed him everyday more than he needs, so he's not under fed. His tail keeps getting shorter. He used to be about 14 to 15 inches. Now he's about 9 to 10. He doesnt swimm as good now. What could this be?

 

[Kenmx10]

I just now noticed a large round bump on his tail.It looks like a bubble. Almost like chicken pox. Only one. He seems healthy. He's very active.

 

[kheebl]

Mine is doing the same thing, and I can't figure out why?