I was running the skimmer air intake from outside the house, then I did the co2 test and the levels are good in the tank, even if you stay in the tank room all the day. The tank is in the basement and I don't spend much time there and the window is always a little bit open. As for air exchange my system is composed by 5 vessels, none of them has a hood or canopy of some sort.
In my case it is not related to carbon dioxide