I’ve never quite understood that. As plants produce O2 when photosynthesising, you’d think that a CO2 injected tank with higher a rate of photosynthesis will also have higher levels of O2.
Yes, that's all correct. Still, adding oxygen often works well.
One point to consider: The more CO2 plants consume -> the more sugars they create -> the more energy they can gain from
oxidizing sugars, which is O2 consumption (= respiration). The latter may happen during night, esp. within
dense stands of plants. (Scientists observed that juvenile fish which normally seek shelter in dense stands
leave these spots during late night and early morning. They wondered why and learned that such spots are - counterintuitively - suffering from
lack of oxygen.)
Second point to consider: Many reduced substances are toxic. (Ammonia, too, is nothing else but reduced nitrogen. But there are more dangerous compounds than that.) These substances move upward from lower, suboxic or anoxic layers of substrate. There's non-zero period of time between reaching border between substrate and water column and their oxidation (= detoxication). The risk is increased when the substrate is covered by dense canopy of carpet plants. Obviously, this dangerous situation is less pronounced if the water column is superbly oxygenated.
==
Of course, I can't provide any exact proof that what I've described actually happens and does harm in our tanks. But I have read what I've described in scientific papers, explored various means of oxygenation and it serves me well. Including Söchting Oxygenator, which is particularly suitable for CO2 injectors because it avoids that classical dilemma "CO2
or O2". You can have both.
