Has anyone heard of really aggressive behavior in zebra danio fish? One of mine severely damaged a golden killifish (that was a little bigger than it) by ramming it in the stomach/bladder area and attaching its breathing fins. Then it attacked the other danio in the tank the same way. The other danio is fine because I saw the behavior and got the bad danio fish into another container in time but the golden killifish probably will not make it and is swimming on its side, grasping for air right now. It can't even eat any food. The golden killifish never bothered the danio fish except in the beginning when it was introduced and the danio fish tried to pick on it. I can put up with one of the danio fish just chasing the other fish but this one deliberately tried to kill both of the other fish in the tank and bite them. Is this normal behavior for a zebra danio fish?