Clean the intake on your powerheads regularly. As your pumps sit in the water, they attract algae of various types. Coralline algae for example will grow on the intakes and outputs of many water pumps and powerheads, sometimes restricting flow. So, remove the pumps ever couple of months (or sooner), soak them in a vinegar and water solution for a few hours, scrub them with a brush, and put them back into the aquarium. The increase in flow will be obvious.