The filter materials determine the quality of the filtration process and the clearness of the pond water. The right combination of filter materials is essential. VT offers four different filter materials with a filter net, sufficient for 3000 litres of pond water. Fill the enclosed filter net with the material and rinse it several times with water before placing it in the filter.The coarse structure of the chemically purified pieces of filter lava makes them particularly suitable as biological filter substrate in all kinds of pond filters. In the many pores of the porous material, many billions of micro-organisms will develop, whereby an ideal filtering of the pond water takes place.