what other fish beat it up? imo quarantine it because it is most likely stressed out and that can lead to ick. its better for him to get ick in the quarantine tank than in ur main tank caus ethen ur gonna have real problems. make sure he has hiding spots to hide so he feels safe. his fins should grow back soon. there is still the chance that when u introduce him back into the main tank that he will be beaten up again, so keep an eye on him.