Mark,
Take this with a grain of salt as I had not had to fight a marine infection yet, this is freshwater experience talking:
If you see ANY result from your medication, keep going. You may inadvertantly help make the bacteria in the infection more resistant to the medicine.
I would check the bottle for counter indicators, but in a hospital tank, I think mela-fix is a great product to 'naturally' boost the medicine you are allready using.
most pop-eye in fresh water at least, can be linked to water conditions (In my experience) Monitor the water in your hospital tank, keep it pristine as possible to help your fish's immunity.
If it takes flake, you might want to try some medicated flake food.
Now, as I said, That is freshwater experience, and salt water is a whole different ball game, so you might want to get some confirmation on that.
I once had a beautiful silver arrowana who's tank crashed while I was gone (my fault, was trying to do to much with the tank) when I got back he had pop-eye. I corrected the problem conditions in his tank, but the eye took weeks to return to form. Some fish will lose their eyes, or never recover. So while I hope for the best, be ready to accept 'the worst'.
(note the arrowana eventually recovered, grew to a size I could no longer accomodate (and since I met my wife and got engaged, a bigger tank was no longer in the picture... so I had to sell him. I am sure some 12 year old bought him and ended his life miserably.)
"Maybe in order to understand mankind, we have to look at the word itself: "Mankind". Basically, it's made up of two separate words - "mank" and "ind". What do these words mean ? It's a mystery, and that's why so is mankind." ~ Jack Handey