The power head is used in conjunction with an undergravel filter. The undergravel filter is a perforated piece of plastic which is at the bottom of the tank under the gravel. There is a tube riser which ends just below the surface of the water with the pump sitting on top of the riser that pulls water from the riser, which forces water down through the gravel. The pump head has a filter and aerator so sediment from the gravel is collected and at the same time the water gets air added.

The same undergravel filter and riser can be used with a simple airstone in the riser so the rising bubbles drag water up the riser which means the same flow through the gravel as by the pump, but much less flow. Aquarium power heads are capable of moving more water than the airstone method which means you can support a much larger tank with an undergravel filter. Most aquarium power heads provide the same function. It generates water movement and circulation in aquariums. However, care should be taken in choosing as differences lie in maintenance requirements and cost.