With a controller, run the CO2
24/7/365. As for bubble rate, between 1 and 2 bubbles per second should be sufficient, and not overdo it. It will get you to the level you need and the controller will handle the rest.
One other thing to remember, with running CO2
at night, it's good to have some type of surface aggitation at night. Reason being, plants at night will stop taking in CO2
, and start taking in O2
. So you will end up with normal injected CO2
levels, and lower O2
levels. The surface aggitation won't hurt the CO2
levels much, but will keep the O2
levels at safe levels for the fish, as well as the plants.