The coarse fliter pad is made of high grade pored foam, so the water that passes through the pad will be cleaned of most larger particales. The coarse filter pad will serve as a biological and mechanical filter, essential to the health of your aquarium. Coarse filter pad are re-usable and last longer than your fine wool filter pad, however your coarse filter pad should be replaced as soon as it loses shape. Failure to do so will lead to particales avoiding the filter process.