You only need CO2 if your running temps over 80. Also you have to not run the circulation fans during lights on or else the CO2 will never amount to enough to get any noticeable growth.
Don't waste the time on a little 20oz bottle. If you want to generate CO2, brew 5 gallons of beer or ferment 5 gallons of wine, but you will have to start a new 5 gal bucket every 2 weeks, so that will take up all kinds of space.
Also, during the first 24 hours the yeast needs oxygen, do not cap the bottle and your yeast will make more CO2.
CO2 is heavier than air so if you set it on the floor of your grow box it will never reach the leaves, also your roots take oxygen and CO2 where they expect oxygen might cause problems.
I brew and have noticed a tiny change, but not enough to justify it for marijuana purposes.
Your light should be on a sealed air system before you do anything like this though or you will just cook the plants.
~GG420