These efficient Biofilters contain fine filter foam for mechanical filtering and two types of filter medium for effective biological treatment. Top inlet , bottom outlet - require gravity-feed back into pond. Includes over-flow and multi-hosetails for easy installation. Pump sizing is given for maximum pond size with 0.5m head. Ensure this suits your application, as if the pump is to supply other features, e.g. fountains, it may require a larger pump size.