My research indicates that fin rot is the visible effect of a fungal &/or bacterial infection, so QT'ing the little guy is a good idea. I moved a cory to the hospital tank when he developed fin rot last week. None of the other fishes are showing signs of having 'caught' it... If it's even contagious. However by isolating him, I'm only medicating one fish as needed rather than the whole tank.
If your betta's solo, I'd do the tank cleaning and pwc
now, use the Pimafix and Melafix at 1/5 the dosage for 7 days then a 20% pwc. Rinse and repeat until symptoms are gone.