An undergravel filter is a basic method of performing biological filtration and some mechanical filtration. There is a tube that is attached to this bottom piece, known as the lift tube. Gravel is placed on top of the filter, and the filter, lift tube and gravel work together to provide filtration.
Water is pulled down to the bottom of the tank, through the gravel.
And then the undergravel plate, then flows back up to the top through the lift tube. The water is again pulled down through the gravel. This allows beneficial bacteria to thrive on the oxygen laden water flowing through the gravel. If you are using a power filter or some other type of filtration and no undergravel filter, you may purchase slightly less gravel. One pound of gravel per gallon in your aquarium should be enough.