Clean, healthy water: nitrite, nitrate and phosphate removal kit for CristalProfi e filter baskets. To use: remove the plastic bag, place the complete kit in the empty filter basket (second-to-last seen from the top), replace the filter basket in the filter. Removes the toxic nitrogen compound nitrite (NO2), nitrate (NO3) and phosphate (PO4), which promote algae growth. Mixture of clay balls and special resin in a bag inserted in the center of the foam. Only part of the water passes through the bag, and the slow passage of water enhances the effect. The main flow of water passes through the foam.