Goldfish help please?

My young female goldfish was attacked awhile ago by my young male goldfish.

I have been treating her fins for finrot cause that is what it looked like, I've been using Melafix everyday and keeping up with water changes and removed the male from the tank.

But now she has red around the base of her tail, with white and some fraying along with scales raised around the base of the tail. She still eats and still greets me when I come into the room, but this is really getting me worried. She's getting sicker, her tail used to be long and flowing but now its all stiff. She's really sweet I want to do everything I can for her. I tried looking stuff up in every fish book I could I havent seen anything like this. Please help.

Extra info: She's in a 70-gal right now should I put her in a smaller one for treatment? she's only 3 inches.