What Is an Undergravel Filter?

An undergravel filter is a simple aquarium filtration system that uses an air pump to draw water through a plate underneath the gravel. The gravel itself acts as a biological filter, harboring beneficial bacteria that consume ammonia and propagate the nitrogen cycle.

Dedicated undergravel filters for large or heavily stocked tanks should use a water pump or a powerhead instead of an air pump, since this allows better water flow. Undergravel filters should not be used in aquariums with aquatic plants, uneven gravel size or sand as a substrate, as these items can inhibit water flow and cause the water around the substrate to lose oxygen that fish need to survive.