Well, from what I've read...(weeding out all the info I think is bunk)...you would use the CO2 when the lights are on only. The CO2 is absorbed/used most efficiently at higher temps, like 85F-90F, which would only be possible during lights on anyway. Plus the photosynthesis requires it. So, yes, when lights on only.

As far as the fan and your tubing goes...you could hook the tubing up to your ceiling with the holes to let it sink into your room. You will want to make sure your exhaust fans are off, of course, so you don't suck out all the CO2 you are introducing. I have read that you should leave your 'circulating fan' on to move the CO2 around the room and allow the plants to use it completely. I'm not sure if this is 100% correct but it seems logical, as long as you aren't sucking it out.

My :twocents:

:rastasmoke:

TGF