It's important to change your carbon blocks on time as they remove many things you don't want in your tank such as chlorine, herbicides, pesticides, and disinfection byproducts. It is also super important to protect the membrane by removing chlorine. As little as 200 hours of contact with water containing 1ppm of chlorine can ruin the membrane. The best way to know when to change your carbon blocks is to test the water. To test them: 1. Obtain a sample of water to test by either disconnecting the tube coming out of the last carbon block or by getting a sample of your waste water. 2. Test the water for total chlorine with either simple test strips or a chlorine checker. 3. If you detect chlorine, change your carbon blocks. Connect with us on Facebook: Check out our pics on Instagram: