Matala is a ridged woven filter media that ia great for both biological filtration as well as mechanical filtration.  There are four different densities available:  From least dense to most dense, we have Black, Green, Blue and Grey.  These pads allow oxygen rich water to flow through it,  providing a great environment for the growth of bacteria, while filtering out solids.  Many different combinations of densities are possible.  See My video on YouTube about Matala for more information.