Im not expert but i guess it depends on what fish you keep.what have you got in there? do they like a lot of flow? i would base my powerhead power on that.hope that helps, im no expert (im 15) so wait for some other guys 2 reply, i dnt want 2 kill ur fish lol.Matt
Got any extra pumps laying around? You could drop one in the tank to see how the fish like it (if it's too strong put a 'T' & valce on it to bypass some of the water.