Not to sure bout this fish but with most schooling fish you need 6ish or more and most of the time if you do add more they do stop being so aggressive. As I say I have limited knowledge of this fish but that is how most schooling fish work. Obviously the only problem is that tank is not big enough for 6 as the minimum tank size is 50 gal anyway. You have choices but I would probably remove the more aggressive one if the other is in danger health wise and try to find it another tank mate.