Power Heads: Power heads circulate water within the aquarium. The function is to aid the workings of under gravel filters. To improve aeration of the water, most power heads include an adjustable air vent . If you use a power head, you don't need to use an air pump to power the under gravel filter. However, you'll still need an air pump if you want to power action ornaments or decorative air stones. A power head can also be used as a stand-alone unit in the aquarium. It does not need to be placed on top of a lift tube and can be put in an aquarium for the sole purpose of circulating and aerating the water. Some power heads come with an attachment that turns a power head into a biological and mechanical filter.