How can CO2 remain 30ppm all night long?

Think about that.
Adding some more surface turbulance will take care of that, but what you are suggesting is that the CO2 level takes over 12 hours to degas and even then, that barely puts a dent in the CO2.

I doubt that's what is going on.
Increase the surface turbulance some, don't worry about it.


Regards
Tom Barr