I have never found that my CO2 concentration drops all the way to ambient values over night. I have quite a bit of surface ripple from my spray bar, just to make sure it does drop a significant amount, but the drop checker is only blue green in the morning, not blue. So, a .5 drop in pH when you add CO2 the next day could very well mean 30 ppm of CO2 in the tank, or it could be more, or it might even be less.