Eheim Ehfisubstrat is a highly effective, economically priced and perhaps the best biological media available to aquarium hobbyists. Biological filtering is based on a natural decomposition of harmful substances using helpful bacteria. They convert ammonia and nitrite into relatively non-toxic nitrate.The efficiency of biological filtration is limited by the media that bacteria are growing on. With over 3000 sq. ft. per U.S. gal. Ehfisubstrat is a specially designed sintered glass. Bacteria are able to stick better to a surface which has a complex pore system. Ehfisubstrat has been specially developed to offer optimum sites for bacteria colonization. The effectiveness of these bacteria is linked to how much oxygen and toxins can flow by. With faster decomposition of toxins compared to other media.