Zebra Danios becomming increasingly aggressive

Hi all, I got 5 zebra danios to cycle my tank along with 3 neon
tetras. The zebra danios have always been very active and it took the
neons a few days to learn to deal with them. All have been in the
tank for about 3 weeks now. However in the last 3 or 4 days the danios
have become increasingly aggressive toward each other. I haven't seen
any nipping but maybe that's going on. They're definitely chasing
each other around for hours on end. All 5 seem to be involved in this
and its definitely more noticible than it has been. The water quality
is reasonably good and hasn't changed lately. Any thoughts on what's
causing this behavior or if I should be concerned?