I have a school of 14 Danios - 2 Zebras and 12 Glow-fish - in a 4 foot long, 33 gallon tank and they usually just school around together peacefully.
This morning, before the timer turned on the lights, I noticed a wild and crazy scene. The males were chasing the females in packs and literally "attacking" them one after another. A group of 3 or 4 males would single out a female and "drive" her all over the tank at break-neck speeds until they would eventually pin her to the gravel and cluster around her. It looked as though they were trying to kill her dead. Eventually she would escape and the "team" of males would either chase her down again, or single out another female to make miserable for a while before moving on to yet another female victim. The females were terrified and this was obviously not a mating ritual - at least on the part of the hapless females. Though this happened to some degree to almost every female in the tank, one in particular withstood the brunt of it. It was disturbing to watch.
Shortly after my turning on the lights manually, it all ended and peace returned, but the females were obviously traumatized for a while.
What exactly was this strange behavior? Has anyone else ever seen this kind of activity take place with Zebra Danios? I hope I never see it again.