Carbon dioxide CO2 is released by heterotrophic organisms (animals, many bacteria) while oxygen is taken up. Autotrophic organisms (plants, algae, zooxanthellae and some bacteria) absorb carbon dioxide during the day and release oxygen in return. At night, however, these organisms also release CO2 while consuming oxygen. Regular fluctuations in the carbon dioxide concentration are the result.
Carbon dioxide is in equilibrium with carbonic acid H2CO3 and also with hydrogen carbonate HCO3 and carbonate CO3 (lime-carbonic acid equilibrium). The sum of hydrogen carbonate and carbonate is approximately also called carbonate hardness or alkalinity. The pH value is important for this equilibrium, as well as temperature and salt content. This relationship is not easy to understand. Therefore, only here is the rule: if I change something at one point, I change all parameters related to this factor as well. For example: If carbon dioxide gets into the water in greater quantities (at night, operation of a calcium reactor, too high CO2 concentration in the air above the aquarium), the pH value drops and the concentrations of carbonic acid, hydrogen carbonate and carbonate change. If, for example, KH former is dosed, the pH value also rises, among other things. If the carbonate hardness drops, small changes in the carbon dioxide concentration can cause large fluctuations in the pH value.
Measuring the carbon dioxide concentration directly in the aquarium water is not useful. If the pH value and especially the carbonate hardness are checked regularly and adjusted if necessary, the carbon dioxide concentration will be in the right order of magnitude.