What I mean is that if you are struggling with CO2 stability, or are having difficulty keeping high concentrations, then it's best to focus efforts on the first half of the photoperiod and not worry so much about the second half. If you are running very strong lighting then you are really pushing the plants and it's necessary to have the CO2 going. After 4 hours, if you turn the gas off, the water stays saturated for a couple of hours after the valve closes and the concentration trails off. So if you have an 8 hour photoperiod and you turn the gas on 2 hours before lights on, you can turn the gas off after 6 hours. So the gas is still running for 8 hours but it's on-off cycle is offset by the amount of time you turn it on prior to lights on.