Hey, powerheads just give your fish some current, or, in your case, mix up the water so you can avoid 'dead spots' in water current. These are areas where there is no current, and therefore no movement, in the water, (hence the 'dead'.) With a 75 Gallon though, I would go with a canister NJ, but thats only my opinion. Other than that, th two Emps would do fine.