I think I'm losing my Purple Tang!!!

[Dee12dee12]

He has a clear bubble hanging out of his anus. The scales around his anus has been scratched off so that area is now grey. I think that the clear bubble which is very small may be part of his stomach. He isn't eating the foods he usually eats but he is picking food off the back glass.
Has this happened to anyone. I'm hoping to save him so any advice would be appreciated. Thanks!!!

I will try to capture a pic.