By turning off the lighting for a 3- to 4-hour period, plants cease strong photosynthesis, and dissolved CO2 has a chance to accumulate to almost pre-light levels from plant, animal, and bacteria respiration, as well as (likely) via diffusion from the air.
The third option is to use a “siesta” or a few hours with the lights off during the middle of the day. Several aquarists have demonstrated that CO2 in many planted tanks is just about depleted after 4 to 6 hours of photosynthesis. By turning off the lighting for a 3- to 4-hour period, plants cease strong photosynthesis, and dissolved CO2 has a chance to accumulate to almost pre-light levels from plant, animal, and bacteria respiration, as well as (likely) via diffusion from the air. When lights switch on again after this period, dissolved CO2 is in abundance for the next round of photosynthesis (see plantedtank.net and p. 179 of “Ecology of the Planted Aquarium” by Diana Walstad for further information). This break or “siesta” during the day has the added advantage of reducing power consumption and total photoperiod time to reduce the chance of algae outbreaks. It also means you can enjoy your tanks in the morning and well into the evening if you work standard daytime hours.