I'd say about 20 lbs.

Anyway, this has to be up there with dumb posts... I hope that guy wasn't serious about not knowing if iguanas breathe or not. Don't lose sleep over co2, there are much bigger concerns than that! You could affect your yield more with a dehumidifier than yeast. As long as you have proper ventilation there will always be enough co2, trust me this planet has surplus.

Also, everyone knows that canaries make the most co2.

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