Melafix is a med, both practically and legally, but there will obviously be times when it isn't what you need. I have tried to ask the API scientists who worked on it if they can say when to use Melafix, and when to use antibiotics, but they said they can't really say, because there are too many variables. The only thing they did say was that fish heal better after the infection when treated with Melafix than when treated with antibiotics, and secondary infections are much less common. There's also no issue with filter bacteria.
When using Melafix you normally only do a water change after 7 days.