What disease does my fish have?

Okay, I re-read this three times and I must be blind but is Ben guessing that it is a goldfish or did I miss something. To be honest to Ben, I was guessing the same thing. I initially though that the cloudy cornea (yup, same as people eyes) could have been the result of an injury, but a scratched cornea should have healed long ago. Considering that both eyes aren't affected and there hasn't been any signs of popeye, my best guess is that it is a cataract. Yes fish get cataracts too. They can be the result of a nutritional imbalance, a parasitic infection of some type, or genetics. Unfortunately there are no proven cures to this condition. Well that is my understanding anyway, but you are free to research this in greater detail if you would like to. There is also the slim possibility of eye flukes, but again I think this should have spread to the other eye. If you look really closely, can you see any wormlike critters or tiny bumps on the cornea? If so, a medication for parasitic flukes could remedy these blood suckers, but I seriously doubt that this is the case. I just am trying to cover all the bases. I still bet it is a cataract, but I don't know if this is causing the abnormal behavior. I can't imagine that it doesn't irritate the fish to some extent though. Although, I have seen fish that had both eyes eaten and they still were able to find food and live for many years although they obviously were more prone to other complications like infection. So hope might not be lost for your fish, but he may remain grouchy and lethargic. The fact that he hasn't given up yet is an indication that he is a fighter. Good luck.