What should I do for the fin rot of two male betta fish in a 2.5 gallon filtered and heated divided tank?

So, I have two male betas in a divided 2.5 gallon tank. It has an under gravel filter and small heater. One is a crown tail the other is a split tail. The water has aquarium salt in it. The temperature is usually 75-80 degrees or so, I think (I'm not at home right now). On of them got a torn fin after an encounter between the two of them due to my not properly inserting the divider (they were both fine and seemed to be at somewhat of a truce on opposite sides of the tank with the split tail having a torn fin). The fin rot seems to been caused by that and now both have some degree of fin rot (due to my own laziness, yes it was stupid, i know that now). I've been reading on different sites but I just wanted some input on this particular situation. I have some, I think Mardel, fin rot treatment at home that I have not yet used. Should I put the two of them in separate tanks for this, I don't gave another open filtered and heated tank so it would just have to be a fish bowl? I gathered from the site I should do a water change but how much? or should I just clean the whole thing completely filter, rocks and all? I can't fit a normal suction cleaner in the tank because it's so small. Any advice you can give me would be greatly appreciated. thanks for your help!