The Katadyn Vario Carbon Replacement (2-Pack) is the replacement carbon for the Katadyn Vario Microfilter. The carbon in your vario should be replaced whenever you notice bad taste or odor after filtering the H2O or every 6 months based on usage. The Carbon in your Vario MicroFilter keeps the H2O tasting New and reduces bad tastes, odors, and other toxins in the H2O.