The Ultra Water main filter starts with BioStone Impregnated Carbon, we then turbocharge it with UltraWater technology ensuring unmatched reduction. Removes up to 99.9% of virtually all contaminants, including Chloramine, Pharma, heavy metals and the toughest to remove contaminants like chromium IV and arsenic. Add Scale Guard protection and it out performs any other filter we know of. Clearly your best choice. Goes on the LEFT side of your H2 Series ionizer. (Rated for 4000L)