Filter media is the material inside a pond filter which forms a barrier that provides mechanical filtration for fine particles of sand, grit and dirt within the pond. The high-surface area bio media provide a place for the bacteria colonies to thrive. These micro-organism colonies provide a natural filtration system of beneficial bacteria. Filter Media also helps keep your pond free of particles thus improving water clarity. From the Oase to the Swell range, Swell UK ensures that your pond is chemical free and provides a healthy environment.