That happened to my goldfish after I had him for six years. It would go away and come back again once and awhile. One day it looked really bad, but he lived 3 years after that. Hope he gets better=] They make stuff at pet stores for red spots on eyes and tails, that can be like fungus I think. Or it's like bacteria. Go to a pet store they can help, and it can go away on it's own. That happened for my fish.