It is commonly called pop eye. I'd talk to a trusted aquarium store or go to a site specializing in fish (www.aquariacentral.com is a good one) for info on how to properly medicate. In the meantime, seperate it from any other fish you have so they don't catch it. Make sure the water is at the proper temperature and ph for the breed you have. A partial water change would be a good idea, make sure to use a dechorinator P.A.T is a good one.
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