Is my problem fish stress...or what else?

i just brought home some guppies and they all seem to be swimming at the very top of the water but they aren't acting as if they are gasping for air or swimming on there side. If this is stress what should i do? I've seen things about covering my tank with a towel and using stress guard. What else can i do to make them less stressed if they are. Could turing off the aquarium light also help? They have been home for about 21 hours now and i left the light off all night.