Eheim's Course Blue Filter Pad is used as a parting layer between the mechanical (Ehfimech) and biological filter media (Ehfisubstrat/Ehfilsustrat Pro). Coarser particle are restrained and are thus removed from the water prior to entry into the biologically acting filter layers. This filter material may be rinsed and re-used. Simply rinse in tank water or dechlorinated tap water to keep it's biological filtering capabilities.