NEED HELP PLEASE!!!!!!

[donkey_mittens]

hi i have a rope fish for about a month and everything was fine until today i noticed a spot on him kinda like he was shedding that was smaller then a penny and now its the size of a toony and hes playing dead hes my favorite fish and willing to do anything to bring him back.... all ammonia ph nitrates everything is what it should be if you might have an idea or maybe something i can do would be very appreciated....

btw all my other fish in the tank are perfectly healthy