Okay, there are a couple of possibilities with that one and those pictures are clear enough to tell for sure. Your fish for sure is very old already. It could be a malignant tumor but something tells me it is not. If it were mine, I'd be treating for a bacterial infection. That looks very advanced. The other possibility is that it could be viral but usually a viral infection will go away on its own. There is no treatment for viral infection even in people.
Your two real choices would be to treat for a bacterial infection or perform euthanasia as wataugachicken suggested earlier.