Alright so I had two bettas in one 10 gal (divided) a while ago. One day when I was at school they broke through the divider. My once beautiful was greatly injured. I bought another 10 gal and kept the water pristine. He still got fin rot. Me being the oblivious teenager I am didn't catch it for a while. Now he looks like a plakat with fin rot... For a while I've had him in a half gal with a little salt and daily to every other day water changes. It's not working as well as I'd hoped. He sits either at the top or at the bottom. Then sometimes I can see him almost freak out and gasp for air. My mother came into my room the other day and asked me when he was going to die.
Right now I'm considering running a salt bath and then putting him into a comfortable 2.5 gal with a cup and a few plants to rest in.