in my experience once it hits around 12" mark it becomes a killer, I mean EVIL. Its not that they kill to eat but just for sport. they tend to either take chunks out of larger fish or bite any smaller fish in half and leave the other half on your tank bottom to scare the hell out of you when you walk by the tank.
we think thats a nice knife..... we also think you do not have to worry about it getting big, enjoy it while it's small because once it gets some size it's gonna kill things....at least we think it will anyway......