Do they do it ALL the time ... or maybe once a day or so? If so ... mine do the same thing. The real judge if they're stressed is do they hide the rest of the time, or do they swim around the tank.
While mine do swim crazily around the water current .. the rest of the time they swimming around happily. You could always get one more (assuming no overstocking issues) to have a small shoal of three ... that's what I'm looking to do.