Bettas, the fin should regrow but if it gets torn up too much too frequently, there are chances it won't. I have a plakat who still has a big chunk out of his tail from a divider mishap. If the fishy has fin rot and that rot progresses to the body, the odds of the fin growing back are slimmer but if the fish is just biting then it should be okay. Since I don't think many fish can bite their own butts.
AQ salt at 1 tsp per gallon as long as you have no snails, shrimp, or live plants.
EDIT: Laki, thanks! I'm not sure, they were on recycled paper bedding at the pet store (same kind I'm using, Carefresh or something like that) but the rats were a new shipment so who knows what they were on before they got to Petsmart. They're both going in at 8 PM tonight. The lady was nice and said "Just to let you know it will be $66 for the exam and $44 for the additional pet." T_T