Guppy Bacterial Infection

Hello Everyone,

I have a guppy with some kind of bacterial infection that I cannot get rid of. It started with one fish getting a fin nip, and turned into fin rot. I treated with Melafix and it seemed to work on all but one fish. I am on dose 6 of 7 with melafix. She cannot swim but stays vertically positioned. She started one very small part of her tail with rot, and after noticing her having strange behavior I confined her with a very large (the kind you catch larger Cichlids with) net to keep her from being picked on. Her tail still doesn't look too bad (only the bottom part looks to have rotted at all, and not very much), but there are now large blood spots on her underside and on top. She wouldn't eat this morning.
Anyone have a miracle cure?
I have thought about switching to a different fin rot treatment, but at this point it is an internal infection and I don't know that I can treat that.

The other fish in the QT tank are active and look to be good, their tails are healing. Water parameters have been fine in the tank, this was probably caused by a heater crash that brought the tank down to 70 Degrees.