Hi Sheridanp,
I'm looking at a bottle of Pond MelaFix made by Aquarium Pharmaceuticals. It contains 5% Melaleuca. Regular MelaFix contains only 1%.
Either one can be used as long as it's diluted properly. Take care not to use too much because it's a strong produce and an overdose can burn the fish. I buy 100% pure melaleuca oil at the health food store and make my own version.
Since MelaFix is just a mild bacterial agent, it will sometimes work, but not always. If you don't see marked improvement in your fish's condition within a couple of days, do a good water change and switch to a more reliable medicine.