The resin replacement cartridges are easy to install and virtually hassle free. You can change the resin cartridges in less than 5 minutes. Simply unscrew the watertight housings and remove the spent cartridge. Then, put the replacement cartridges inside the tube and reattach it to the body of the unit. You'll receive two 20-inch replacement cartridges that will de-ionize approximately 300 gallons of water.