I think it would depend on the size of the power heads. What size are they? From someone else on this board they said you should have power heads that are 6-9 times the capacity of your tank. So if you have a 30 gallon you should have roughly from 180-270(gph) to keep things moving. I was also told to have 2 of them and point them to the center. I am not sure if this helps or not, but this was what I was told yesterday.
--Andy