This type of filter involves a few long, plastic tubes which remain upright in the aquarium and are attached to a plate on the bottom. In each tube is an air stone attached to an air pump. As the air bubbles rise, a current is generated which continuously brings water from the aquarium, through the substrate and to the top of the tubes.
Under Gravel Filter. Perforated plates rest on the aquarium bottom and support the gravel substrate. Using lift tubes and power heads, water is pulled down through the gravel, up into the tube, and returned to the water column. Not usually recommended.
Under gravel Filter. Basically it is a perforated plate or series of joined perforated tubes which is placed under the aquarium gravel. Water is drawn through the gravel, through the plate or tubes, up through a lift tube, and finally recycled back on top of the aquarium water.