Matala Filter Mats are lightweight, durable and easy to cut. Matala Filter Mats are available in four densities. Black ( Low Density ), Green ( Medium Density ), Blue ( High Density ) and Grey ( Super High Density ). Low and medium density Matala Mats are perfect for mechanical filtration, while the high and super high densities maximize surface area for biological filtration. Matala Filter Mats provide the highest surface area of all filtration products on the market and are made with a special coating that allows water to pass easily through it without channeling. For best results, pair both medium and high density mats together.