Popeye in fish have a couple of different causes. Is just one eye involved?
If one eye is affected, the fish is still eating, and you cannot see air bubbles in the eye, it may heal on its own (but will take up to a month). If you get signs of infection: redness, worsening of symptoms, then Erythromycin, Maracyn 1 would be indicated. This sometimes impacts your beneficial bacteria, so you need to monitor the ammonia after dosing.
As said, Melafix is an herbal tonic. It has been shown to reduce parasite loads in experiments, but the dose was different and "reduce" is not the same as "cure". Melafix is "patented", but the patent is for the emulsifying technique used, not the medicinal properties.
I do not use it myself and it is not part of my formulary. There are too many cases where it didn't do anything. The simple fact that it won't hurt beneficial bacteria will indicate that it also won't hurt pathogenic bacteria either.
Jay