External filters abide by the same principle as internal filters, except that the water is sucked into the filter outside of the aquarium, is then cleaned and returns back into the aquarium. The function of the external filter is to maintain the ecological balance in the aquarium which nature does by micro-organisms (bacteria). These transform organic waste into simple inorganic substances. We carry external filters of all sizes. We carry all the major brands including JBL and Fluval.