That sounds about right. It's either the plants using up the CO² or some leaking or a combo of the two. You mighty be happier with 1800 ppm then refill at 1500-ppm. Try to hold it for 6 hours then exchange the air in the room. It should only take a few min. Then start the CO² up again until the lights go off then exhaust the air one last time for the day.

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Peace
Zandor

If you go cheap you grow cheap. - Me