Diana Walstad (2015) 

Carbon dioxide (CO2) levels in natural ecosystems often limit submerged plant growth. CO2 levels fall in the morning during rapid plant photosynthesis and are depleted around noon. They are recharged at night by the respiration of fish, plants, and bacteria. The cycle repeats itself the next day.

In 2009, I decided to see what the CO2 levels actually were in my aquariums, which depend solely on natural sources of CO2. The resulting data was more interesting and useful than I expected. I found that CO2 levels were remarkably reproducible day-to-day and week-to-week. Each tank seems to have its own daily pattern of CO2 uptake and regeneration.

I used CO2 measurements to gauge how my plants were actually doing. I gathered evidence suggesting that a Siesta period—lights off for a few hours midday—might benefit the plants. I learned that window light—contrary to my previous assumptions—was not fueling that much photosynthesis.