Try something bad enough to hold its own but not particularly aggressive, like an armoured catfish of some sort. It is a little risky, an angry catfish... plecos have eaten piranhas and others...and many an oscar has fought a pleco until they both died, and I have heard of fish biting through catfish armor, but it might work, if the catfish is a better fighter than the flowerhorn.
Try snails, if you have enough, they'll reproduce faster than your FH can pick them off.