Bowfin are like pickerel in their temperament, in the sense that they don't go in for fin-nipping, chasing, and other small-scale aggression. When they attack, it is with the intent to kill and eat, and they are very good at judging when another fish will fit into their mouths. So, I agree with the others saying that you should avoid new tankmates for now and plan on upgrading the bowfin's tank at some point. They don't need a huge tank, but something with a 2' X 6' footprint would be good. Then your 90 can be a multi-species tank.