Undergravel filters are a time tested method for efficient biological filtration.
Undergravel Aquarium Filters may be considered obsolete by many modern aquarists, or considered old technology, but there are still many people who use them. The primary use for an undergravel filter in an aquarium is for biological filtration. Undergravel filters consist of a perforated plate that sits underneath the aquarium gravel forming a void between your aquarium gravel and the bottom of the aquarium. Riser tubes connect the void underneath the gravel bed to the top of the open aquarium. Water is either drawn up the riser tubes using an air pump and air stones, or sucked up the riser tubes with the use of a powerhead (small submersible pump). This process forces the aquarium water to flow evenly through your gravel bed, and constantly circulated throughout the aquarium. Bacteria colonizes the gravel bed providing excellent biological filtration for the aquarium as the water passes through.
Undergravel Filters are typically used in freshwater aquariums, and are available for most sizes of aquariums. If using airstones to operate the filter, they should be replaced regularly to maintain performance, and gravel should syphoned routinely to prevent buildup of debris in the gravel.