This Hose Ready Filter comes with an internal cartridge that should be replaced on a regular basis. The frequency depends on the amount of chlorine in your water and the amount of water you use. For easy scheduling you can predict that they will need to be replaced annually. However, for more accurate readings you can use a simple pool test kit strip to see if there is chlorine present and replace the internal filter when the strips start to show chlorine readings. Search "Replacement" on our store to find the cartridges.