Kole Tang on it's last legs

Now hes out front. Hes way beyond repair. The fish is now just too weak to get away and my cleaning crew is hunting him down. There are chunks of his skin and flesh missing. I don't know what happened. A couple weeks ago he was fine. Can anyone help diagnose this. He slowly wasted away even though eating and became very emaciated. He then started losing his color. He started coming out less and less. In the last couple days he stopped eating, and now he is too weak to keep getting away, and my shrimp and crabs are eating him alive. I can't even watch at this point. His fins are a little beat up, and theres a patch of his flesh missing on his tail that is being eaten as he desperately tries to swim away. This is the saddest thing I've seen in my fish keeping career. Anyway, any diagnosis?

 

Well, now hes just lying on the sand waiting for the cleanup crew to take over. I killed the lights so he'd have some piece. I can't piece together any idea of how this happened. He must have contracted some disease or had some problem internal problem. Again, if anyone has any ideas of what happened, I really want to figure it out.

 

This fish is long gone, but I'm still wondering if anyone has any idea of what could have happened.