I have to be truthfull that i didn't read through any of the thread yet, lol. If you don't have livestock in there you might as well leave the CO2 running 24/7 if you have a big enough CO2 tank. It will cause less fluctuations, as long as the lights are on a timer should be ok with just the plants. You might be able to reduce the bubble count of your CO2 also. I've not really used CO2 much so i don't know, i've only used the upaqua style CO2 setup.