Eheim ehfisubstrat is a highly effective economically priced and perhaps the best biological media available to aquarium hobbyistsr biological filtering is based on a natural decomposition of harmful substances using helpful bacteria they convert ammonia and nitrite into relatively non toxic nitratethe efficiency of biological filtration is limited by the media that bacteria are growing on with over 3000 sq ft per us gal ehfisubstrat is a specially designed sintered glass bacteria are