Filter Replacement Kit With RO Membrane For RainSoft UF50, 21179 Reverse Osmosis System, DOES NOT FIT ULTREFINER II... INCLUDES: STAGE 1 - 1x Carbon Filter Replace Every 12 Months STAGE 2 - 1x RO Membrane TFC 50 GPD Replace Every 24 Months STAGE 3 - 1x Carbon Filter - Replace Every 12 Months. May need to be replaced more often than indicated depending on incoming water contaminant levels and volume of water being treated.