They only take up large quantities of CO2 during heavy photosynthesis, so if your rain system is on in the dark, you're mostly wasting gas.
This is an obvious dilemma for ventilation. You want to vent heavily during the same period that you want CO2. But then, if you have CO2, you don't have to vent much (or at all, really). It worked ok for us to put a carbon filter in the room outside the tent, cool the light with a duct to a fan outside the room. So the carbon filter takes up the air from the room and uses it to cool the light, which then exhausts, only when the light was on. This also limits the opportunity for stuff like bugs and mold spores to get into the tent, and lets you fill the tent with CO2 to your heart's delight... The only problem we had was too high humidity in a sealed-ish tent.