Popeye is typically stress induced. Check water quality, and any other form of stress possible in the tank, and remove the cause. Other fish (especially conspecifics) are the usual culprits... How to fix it? There's no sure fix for this type problem... When I added my Sailfin Tang it was being harassed by my yellow tang, so I removed the Yellow tang for three weeks and placed him in a "time-out" of sorts in the sump. It did the trick. They both get along perfectly now... Just some things to think about.