It would help to know the following; water chemistry: NH3
, pH, Salinity, Temperature, etc. along with size of aquarium, filtration, and decor (liverock, substrate, etc). Pop-eye usually stems from poor water conditions in addition to abrasion thus causing a bacterial infection. Not to say you don't keep optimal water conditions, but as I said the 'usual.' I personally do not like Melafix as it is far to weak compared to other solutions. Depending on the size of the snapper, you could remove it to a QT and treat or if too large to remove, feeding an oral medication (Enrofloxacin/Baytril).