Ammonia is the #1 Killer of Our Critters
I think many folks turn the CO2 off at night, I suppose the reasons run the gamut, from saving money to a misunderstanding of the CO2, O2 relationship in water.
I think that since fish that live in CO2 enriched waters, live in them 24-hours-a-day, we should mimic nature to the degree we can.
The greater reason is the pH, ammonia/ammonium relationship.
I think people have a tendency to overload (biologically speaking) their systems, the filtration systems are geared to handle one level then suddenly the system changes.