Betta Rescue

I was at Pet-co Yesterday and thought why not check out the Betta Dept. I love Betta fish, But they are NOT the fish for me. Okay back to the story.
So I went over to look at the Betta fish and saw the most Run down, sick, Dying by the second Betta. So I took him up to the register and bought him( out of total Love for animals). He is currently still in his little pet-co cup and is medicated with mella-fix. His whole back tail is gone( fin rot I'm guessing)
and he has a big puffy white bump on his head. Really I thought he was a lost cause but now he is doing somewhat better. I really thought about not getting him but knowing people at pet-co they would either Kill him or let him suffer so apparently I am the Betta fish police. Plus, Petco has a 30 day warranty on all fish so I was not taking a total chance. Anyways, Any suggestions? Help would be greatly appreciated! If you have anything to say please post a Reply!!

Salt! Tsp/gal. Change water daily!!! And keep dosing with melafix.

I have salt! I shall use it thanks