Hi Bev. No windows are opened in the room that houses the fishtank. The 1 difference between all of those readings is that the morning reading is taken after I wake up from my sleep so there will be added CO2 from me sleeping in the room. In the other readings, the room is empty of people. I would imagine my sleeping there would contribute a few percent but will be much less than 10%.
What I can do is measure the oxygen levels of the air in my room in the morning after I wake up and see how far off it is from 100%. If the measurement taken before I sleep and after I wake up are nearly the same, then we can conclude that the CO2 from me being in the room all night is not significant.
One other note is that my DE MH bulb is now 14 months old so a new bulb might give me better performance. I hope to switch over to a SE HQI bulb next using the same Icecap HQI ballast.
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