CO consumption value on a tooltip and info pane constantly increasing until you load.
Tooltip shows some consumption rate (sometimes unrealistic) but actual amount of CO stays the same. It does not consume CO just pressurize it into corners.
Edit: Double-checked. It was a tooltip bug. Terrarium actually consumes CO but the number so small (~170g/cycle) that it hard to see the difference then there is large amount of CO in a tile.
Check algae terrarium tooltip after prolonged game session.