IsoTwist 450-R filters: Stage 1 - IsoTwist Sediment Filter - 1/4-turn 5-micron poly-spun cartridge Stage 2 - IsoTwist Carbon Block Filter - 1/4-turn 5-micron carbon block filter Stage 4 - IsoTwist Carbon Block Filter - 1/4-turn 5-micron carbon block filter IsoTwist Membrane Replacement Compatible with the 4 and 5 stage system 50 GPD Restore Shower Filter Replacement Easy install "pop-in" cartridges Last 6-9 months. Three stage filtration. STAGE 1: Polyester Pre-FilterTraps particulate matter that would otherwise mix with the medias and reduce their effectiveness. STAGE 2: KDF-55This stage destabilizes the free chlorine and turns it into a harmless chloride. STAGE 3: Crystal QuartzThis stage "energize" the water by reducing the water's surface tension giving the water a "lighter" feel.