#platy fin rot

Hi all, Am a newbie here after some help with my platys. They appear to have a white fungus on their tail fins which I have been treating. However they haven't picked up at all. One now looks like she can't close her mouth and is gasping. Can anyone help with a quick acting remedy? I hate...

Here is the situation, I believe my teenage platy has fin rot as well as my daughters betta. I have a 36 gal tank & my daughter has a separate 5 gal betta tank. Her betta used to share his tank with another betta (with a divider) & came down with fin rot. We removed the other betta & got him...