m0useanswered grow question a year ago Its kinda pointless to use CO2 unless your grow room is sealed up, otherwise its just going to be venting out, and if you are venting out into your living space higher CO2 is not ideal.
You can use CO2 at any point in your grow. if you only have a bit of it, I'd use it during the flowering period, If your other environmentals are in check the plant will drink more and can take more light while the CO2 is in use, but this is not really something for beginners. I've never used CO2 in my grows and I don't think I ever will. just seems pointless for me as I do not have a sealed tent.
Good Luck!