undergravel filter

a form of aquarium filter consisting of a perforated plate placed under the gravel on the bottom of the aquarium. The plate has several uplift tubes terminating at or near the water surface. The substrate acts as the filter bed as water is drawn down through it, into the plate and up the tubes. A reverse flow system also works where the water rises through the substrate. The filter bed should be 6.0-7.5 cm deep and the grain size should be 4-5 mm for efficient filtering