This 4 Bay unit comprises of a first stage Particle Separation System (PSS), with Bay 2 being fitted with mechanical brushes, Bay 3 acting as a main biological bay with its “Flocore” medium (or alternative Japanese Matting at an extra cost), and Bay 4 serving as the secondary biological/mechanical stage - incorporating foam and the highly proclaimed aqua rock/alfagrog. With 4 bays offering the best of all worlds this unit is highly recommended.