I know this is a controversial subject, but IMHO CO2 is a waste if you don't have all the other conditions ideal. And it favors growth of fungus. It would be the last factor I played with, get everything else down first. There is a lot of mythology about CO2. Plants can derive all they need from atmosphere unless they are overcrowded and air-stagnant. And those would be the factors to address first if that were the case. Having said this, I do build little yeast CO2 generators for my rooms and its admittedly fun to see them bubbling away. But I seriously doubt they're really factoring much in the overall growth. I agree with ranger that dry ice is the most efficient in terms of money and effort. Just my 2cents. Happy growing.