Two parts yeast, one part water in a Ten gallon bucket will eventually make enough co2 for a 4x4, diy co2 is cheap and does work but you do need to go big and go bigger than you think lol, but folks over estimate the effectiveness of these "exhale" bags by a lot... And no one calculates how efficient their exhaust is... I actually used tnb in my grow... Wasn't bad at all, co2 definitely helps especially during veg, but only those plants kind of close to the co2 because I had a 1300m3/h exhaust fan going... Co2 is a yes in my books, and I'm not convinced it's a "limiting factor" that it only works when everything is perfect, it does help regardless