melafix and bettafix contain the same active ingredient (melaleuca), and neither is strong enough to cure or stop rot. there are antibiotics can can be used to treat rot, though.
how often do you normally change the water? keeping the water superclean will help with any disease, but we need to figure out what is wrong before we can treat it. try to look close, maybe with a flashlight, and see if they are holes or white spots. if they're white spots, are they raised, like a grain of salt?