'ello all! Another upsetting post

About a week and five days ago, my much-beloved male Paradise fish, Bear, died from an infection while in a quarantine tank. About a week ago, I bought a new male, Cape, to replace his unfortunate predecessor in my 29 gallon tank.
Sadly, I have learned the hard way NOT to pick the dominant PF in the dealer tank. Cape has turned out to be a rogue psychopath, quite unlike all my prior PF's...he killed a zebra danio without me knowing, tried to kill several others, and I spotted him yesterday turning on my new long-fin platy (who my female PF, Pearl, has completely ignored, and who Bear would probably have ignored knowing him). Fifteen minutes of chasing with nets later, Cape was captured and transferred to a quarantine tank until I can determine what to do with him (at this point, he is most likely going to be replaced with a more docile, typical specimen).
Anyhow, here's what I am asking: the platy, Sara, had her fins reduced to little more than fin rays, and now has a white patch on her back near her tail. I need advice on how to treat the unlucky fish.