Why cut down the hours of CO2? As long as there is light the plants will use CO2, and the lack of CO2 will be a problem. Granted that the amount of CO2 in the water doesn't drop to zero just because you shut off the supply, but I don't see what advantage there is to shutting it down more than an hour before all of the lights go out. And, there is certainly an advantage to having it on before the lights come on at all.