The Fluval External 306 Canister Filter features a multi-stage filtration system that utilizes the entire canister volume for radically increased water flow efficiency. Its mechanical stage features vertical twin foam pre-filters to increase flow and reduce maintenance. The chemical and biological stages are housed in independent modules. This means you can change filter media independently and ensure a constant level of all-important nitrifying bacteria. The multiple modules allow a vast array of filter media options for maximum versatility.