If you want to know if you have adequate CO2 in the water, and are using pressurized CO2, you should measure the pH of the tank water just before the CO2 is turned on. Then measure the pH when it is at the lowest level, probably just before the lights are turned off. If the pH is 1.0 lower than it was before the CO2 was turned on, you have about 30 ppm of CO2. If it is 0.8 lower than it was before the CO2 was turned on, you have about 20 ppm. If it is 1.2 lower, you have about 50 ppm. That isn't very accurate, but it is more accurate than using the pH/KH/CO2ppm tables.