AstralPool Viron Active Glass Filter Media is unique in that it is 'activated' by coating the glass with a negatively charged layer during the manufacturing process. This attracts organic molecules and impurities which improves the clarity of your pool water over sand or conventional glass. The negative charge is called the Zeta Potential and can be measured in millivolts. Along with chlorine or other sanitisers, the 'activated' glass quickly traps organic material and is rapidly oxidised by the sanitiser. This prevents the formation of biofilm, bacteria and algae in the filter bed, allowing continuous hygienic filtration.