The particular composition of the materials in these shaped filter pads allows thorough water filtration. The whole of each mat takes on water evenly & physically traps both coarse & fine particles Within a short time of installation bacteria will begin to breed in the porous foam promoting intensive biological cleaning Foam pad (blue) Highly porous phenol-free filter medium for physical cleaning & biological toxin breakdown. Washable & reusable Fine filter pad (white) Special filter wadding used as a final filter layer to trap the finest particles. Must be replaced every time the filter is cleaned