Cories do that. They'll do absolutely nothing for quite a while, then for no apparent reason, zoom around the tank for hours. Every once in a while I'll hear a *blurp* from my tank, and it's my cory, after having zoomed around for a bit, almost jumping out of the water to take a gulp of air, then zooming around again. I can't wait to add another couple of cories when I get home this weekend and see how the dynamics change. Will be fun!!!