This filter set includes five replacement filters for the Quick Change (PROQ-550) 50 gallons per day (GPD) Reverse Osmosis System The 5-filter kit includes all filters for the PROQ System Stage 1: 5 micron Sediment Filter (QSF-25003), which has high chemical resistance and removes dirt, sand, as well as rust. It removes any suspended matter (>5 micron) from water, thus protecting the other filters Stage 2: Granular Activated Carbon (GAC) Block (QGA-25002), which removes chlorine and other organic chemicals Stage 3: 10-micron Carbon Block Filter (QCB-25001). This carbon filter takes out residual chlorine and organic chemicals before water goes into the RO membrane/filter Stage 4: 50 gallons per day (GPD) reverse osmosis membrane filter (QME-25050) Stage 5: Inline GAC filter (does not include adapter end fittings) All five filters are to be changed once a year and are disposable after use To change the filters, turn off the tank and feed water valves before replacing the filters All filters are 2.5'' x 12'' and only compatible with PROQ Systems