Black Berkey Elements address over 200+ typical contaminants that could be present in tap water and raw, freshwater sources like rivers and ponds. We recommend replacing Black Berkey Elements after 6,000 gallons, depending on the quality of influent water. Each additional pair of Black Berkey Elements (sold separately) adds an additional 6,000 gallons of contaminant reduction to your system before replacement is recommended. Berkey PF-2 Fluoride and Arsenic Reduction Elements are designed for use in conjunction with Black Berkey Elements to adsorb fluoride, arsenic(V) and pre-oxidized arsenic(III), and other residual typical heavy metal ions. We recommend that each pair of Berkey PF-2 Fluoride and Arsenic Reduction Elements should be replaced after 1,000 gallons of use, under normal conditions. Black Berkey Elements and Berkey PF-2 Fluoride and Arsenic Reduction Elements are backed by independent, third-party testing.