in a gallon bottle i put a packet of active dry yeast and a few cups of sugar, fill it with water, and poke a hole in the lid
when i put the lid on and shake the bottle (with a finger over the hole to not spray shit everywhere)
a ton of c02 is created and it'll almost push your finger off the hole. i shake it everytime i check in on the grow (prolly 2x a day) and even if i dont itll contastly make an amount of carbonation thats let out as slow c02.
i put it right under my intake and i figure its just enriching the air pulled through the system. i donno how effective its been, but it helped he continue with rapid growth when the temps shot up a while back. nothing scientific but it couldnt hurt.