The JUWEL filter sponge coarse serves for the mechanical and biological filtering of your aquarium, making it an essential component of the JUWEL filter system. The coarse pore structure gives a large surface area, guaranteeing relatively high and even water flow velocity. This means that it is particularly suited to biological filtering in the aerobic area. The open pores of the JUWEL filter sponge were made without the use of chemicals, meaning that there is no residue.