Cosmo actually passed away. I had him for 2 years and he had fin rot when I bought him. I tried daily water changes for 10 days but no improvement. So I tried salt and daily water changes and that worked wonders. He never had fin rot after that. But I will take your advice and not use salt on my other 2 bettas. (A female and a male :) ).
AQ salt is okay for fin rot, although I just cure fin rot with daily 100% water changes and 80 degree water and only use AQ for open wounds. But yeah, I would stop using AQ when you do his water changes.