My fish has an eye problem.?

well I looked at the pic and it looks like its just because of the breed of fish or if it isn't the kind of gold fish that I am thinking of than he might have Pop Eye you can go to a petco and buy this. Brand:API Name of product:MelaFix it treats pop eye and you want to get it because if it is pop eye than after it dyes if it dosn't get treated than it will spread to the other fish and it will kill them so get the MelaFix and it says that you should remove any carbon from the filter (if possible) and put in 1 teaspoon per gallon of water. and it says to reapeat for 7 days and on the day after the last treatment than you should change the water and it says to take out 25% of the water when you change it.