The Seachem Matrix™ is a high porosity biomedia that provides efficient biofiltration for the removal of nitrogenous waste. Every Tidal™ filter comes with a bag of Matrix™ to get you started. Matrix™ for the 35, 55, 75, and 110 units comes pre-bagged in Seachem Zip Bags, which feature durable seams & a corrosion-resistant zipper.