JadaBlu, heavy salt will not affect the good bacteria. Chlorine will kill it, PP will kill it, but salt will not.
Tail or fin rot is usually a secondary problem, caused by aeromonas bacteria, and a healthy fish in good water conditions shouldn't have it, but bettas stress so easily, and have such lovely fins :( they suffer it more than most fish.
If you really want to be sure, kill all the bacteria and start over. If you think the other fish are healthy enough, go ahead and use it to seed another filter.
Last edited by lovemyfish99; 04-26-2013 at 12:43 PM.