Aquarium Power Heads
Power heads are typically used to improve the water flow through an undergravel filter. Using a power head with an undergravel filter can allow you to keep more fish without getting a bigger tank. A power head is used to circulate the water in your aquarium. Its main purpose is to help the undergravel filter do its job.
When used with an undergravel filter, a power head is placed on the top of an undergravel lift tube. It "powers" the undergravel filter by pulling the water down through the undergravel filter and up the lift tube where it is expelled out onto the surface of the water. Most power heads include an adjustable air vent to improve aeration of the water. If you use a power head, you don't need to use an air pump to power the undergravel 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.