I have a good suggestion for you. Maybe you should try a clownknife fish at about 12" long. The flowerhorn might beat up on him during the day but after one night. I guarantee that the flowerhorn will calm down. I only say that because clownknife fish are nocturnal and during the night or when the lights are off is when they are most aggressive. Give it a try. They also reach lengths of up to 3 feet long.