Looks a lot like brooklynella. The only med I know will help is formalin. Formalin can be hard to find. There are other meds that contain formalin though. I forget the name, but I think one med containing formalin is even Wal-Mart has it. Assuming this is brook, I'm afraid your chances of saving it are less than 50/50. Brook is very contagious and deadly, you still may have time to save your other fish.