The point of a pH monitor is that it will tell the solenoid to shut off CO2 flow at a certain point. In order for this to be productive, you need to know at what point your CO2 level is acceptable. The easiest way to do this is with a drop checker with a 4dKH solution. With pH indicator mixed with 4dKH water, the solution turns green at about 30 ppm of CO2 in the water column. Past 30ppm, the solution starts turning yellow. Animals living in the tank will start to suffer at some point beyond 30ppm The pH drops depending on the decrease of CO2 content and degrees KH. When you hit a point that you're happy with, you set the monitor at that pH level. When the monitor detects a pH at that level, it shuts off the CO2.