External filters are positioned outside the tank and draw water into the filter through an inlet tube and return the water via a return tube which can be fitted with a spray bar to increase aeration. They avoid the need for an unsightly filter inside the tank. One problem with external filters is that they require priming before use, although some newer models now feature self-priming devices. External filters offer much larger media capacity than internal filters and are therefore the perferred choice for larger aquariums. They normally incorporate areas for several media types so the media can be tailored to suit the requirements of individual tanks.
