I've never used CO2, but have read that the optimal level is 1600 ppm.
So I Googled "excess co2 in plants" and
here is one item I got.
Quote:
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ll Things Considered, February 15, 2006 · Scientists say more carbon dioxide in the Earth's atmosphere may be influencing plants in an unexpected way. More carbon dioxide makes plants grow faster but it also makes them transpire -- or sweat -- less. The result could be extra water running into rivers and oceans.
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I would assume what is true globally - plants transpire less - would be true in the grow room.
