I am sure you mean "ph controller", not "co2 controller" . Low KH means small amount of CO2 can produce large ph change. In a sense, the controller is even more useful here to make sure you do not have a ph crash. The accuracy of your ph controller is not dependent on your kh, it depends on the resolution/calibration of its ph probe. However, you are better off increasing KH to 3-4 degrees so that you have sufficient CO2 for plant growth.