Ritalin or valium might work. No seriously, live breeders can be very aggressive. I’ve lost several fish due to harassment and I have yet to find a good solution. I’ve had three males in a tank with no problems and just take away one male and BAM! Suddenly the other two males are competing for dominance. I’ve had mollies and platys fight. Swordtail males can be really bad. I’ve had a male guppy that drove my molly females nuts; he saw them as really large female guppies. The only real solution I’ve found is large numbers of their own kind or to impose the death penalty. I know that sounds mean, but some fish are just genetically predisposed to being jerks. I’ve traded some trouble fish, but culling overly aggressive fish is no worse that destroying a fish with a genetic physical disorder.