This Biostone Carbon Filter is for Nexus, Alka-Blue and Smart water ionizers. It is the best quality, 13-stage, single-body filter that you can buy for use with city water. This is the filter that comes installed in most Nexus & Smart water ionizer machines. It can trap a wide variety of contaminants while allowing beneficial minerals to pass through. It uses the highest quality NSF-certified filter media available. It is designed to filter out heavy metals and the most common impurities in city water. The "Biostone" (tourmaline) is designed to reduce the surface tension of the water, naturally, prior to ionization in order to stimulate the best possible ionization.