Black Ghost Knife Deteriorating Skin?

[Nyk]

Our black ghost knife's tail started looking like it's deteriorating near it's tail. There is a small piece of skin hanging from his body.

Couldn't get a good pic of him..hpoefully someone can see what I'm talking about to help our bgk!

 

[WILDER]

Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.

I would issolate him and use a bacterial med.
The skin on the body has he damaged it on something sharp in the tank.
I can only see the tail fin in the pics.
Where on his body has the skin be stredded.