A severum only tank wouldn't be a bad idea either. Personally I never had any problem with severums and aggression but that was probably do to them being in with bigger and more aggressive fish.Sorry to hear about your troubles... I acquired a pair of red spot’s to help get my bass to eat pellets... they have attacked all their tank mates ranging from 6” p bass to a 16” TSN ...
They live solo now in a 75 and attack my hand whenever I try to do maintenance in the tank... severums...
Maybe look for a new tank for the sev... he’s in a bucket?