Looking at the pic you posted of the tank in your other thread (nice tank btw!) I would suggest some sort of tall structure in the tank to block line of sight from one end to the other. If you like real plants and don't mind them not being of the same biotope, I would look at something like an Amazon Sword. This will provide tall, wide leaves that the fish being chased can run behind or stay opposite from the fish that is continually harassing them.