I have a single bottle, which I rotate with a pre prepared mix every 2 weeks an hour before the lights turn on, the run rate is 1 bubble per second, feeding via a ceramic difuser. Works fine and co2 drop off is minimal. I have a ph indicator in the tank too, as a safely check. I run mine 24 hours a day but have a little surface movement as the water level is flush with the HOB outlet.
I heard that a number of people say your water surface should be totally still but mine has a good movement and I hold a pretty steady 30 ppm CO2!
Defo run an air stone at night if you have no agitation on the surface.