To filter the large debris out of the pond water, the Filtra-Sub material is specially suited. The large pores are not easily clogged and therefore, the filtered water passes through easily. Through the structure of the material, it is also an ideal attaching substrate for micro-organisms Alfagrog is ideal for use in any biological filter system. It is very popular among koi keepers and commonly used in box or bay filters. Many pond keepers opt to swap plastic bio media for this material for a longer life and better results.