Oxygenation by plants.
The problem is that the amount of oxygen produced is related to the
light input. Low light equals low oxygen output. Also, if the lights go
out at night - those same plants will be consuming oxygen. Lack of
oxygen is usually not the problem - the increase in carbon dioxide is
much more likely to make the environment unpleasant. Lots of water
containers about, changed frequently, will mop up a lot of carbon
dioxide and very little oxygen (different solubilities) but then (maybe)
you will have a humidity problem. It's all swings and roundabouts.
Peter, Reading, UK.