The PH controller is just one way to control the CO2. Even with the PH controller you need a needle valve to adjust the bubble rate - if it is too low, you will never reach the PH you select, and if it is too high the PH will overshoot by too much when the controller shuts the CO2 off. You still need to periodically measure the KH and make sure the PH you have selected gives you the ppm of CO2 you want. If the KH varies, the ppm of CO2 for a given PH will also vary. Personally, I prefer to do without the controller, and just manually set the bubble rate so it holds the tank at the ppm of CO2 I want, then use the solenoid to shut off the CO2 when the lights go off.