Note that - yes you may have a problem with PH movement due to excess CO2 at night but there is no competition between Oxygen and CO2. High C02 levels do not result in low oxygen levels.
I have noticed that it is normal for some fish to hang around near the surface when its dark, I guess because in nature they are immune from attack by birds etc and therefore do not need to go deeper.
I have noticed that it is normal for some fish to hang around near the surface when its dark, I guess because in nature they are immune from attack by birds etc and therefore do not need to go deeper.
