Can you post any photos of it? What are the water params in the tank she came out of? Other tank mates? How big is the main tank? What temp?
The more info you can provide about the main tank, the faster we can help determine what caused the problem and how to best fix it before it happens again.
Without photos, it's going to be impossible for us to offer any advice on how to treat this... and yes, quarantine tank is the best thing you could have done... not so sure about the Melafix. Not a good idea to add medications unless you know what you're treating for.