The JUWEL fine filter sponge is primarily used for the biological filtering of your aquarium, making it an essential component of the JUWEL filter system. The fine pore structure gives a very large surface area, guaranteeing slow and even water flow conditions. This means that it is particularly suitable for biological filtering in the an aerobic area. The open pores of the JUWEL filter sponge were made without the use of chemicals, meaning that there is no residue. Characteristics of the JUWEL fine filter sponge Optimum biological filtering characteristics Homogeneous structure, guaranteeing even flow conditions No chemical residue Suitable for use in freshwater and saltwater aquariums