Carbon dioxide

Carbon dioxide (CO2) is a gas that organisms (including pond fish) exhale when they convert glucose into energy with the help of oxygen (O2). Glucose is formed in these fish from the food they ingest and thus ensures their function (growth and movement). Fortunately, plants, under the influence of sunlight, do the opposite, converting carbon dioxide into glucose and oxygen. Otherwise, there would be a shortage of oxygen on Earth and an excess of carbon dioxide. Perhaps you remember this phenomenon, called photosynthesis, from high school biology classes? Carbon dioxide is essentially the result of the combustion (digestion) of organic matter (food).

Greenhouse gas

Unfortunately, humanity produces more carbon dioxide than plants can convert back into oxygen. Not only does the digestion of food lead to CO2 emissions, but all forms of combustion contribute to this. Consider, for example, the combustion of fossil fuels in industry and transportation. The amount has been increasing alarmingly for decades. CO2 has the unfortunate property of absorbing (storing) infrared radiation, causing global warming. This can have serious consequences for the planet's flora and fauna.

CO2 and pH

The CO2 released during the digestion of food in your fish's bloodstreams lowers the pH (acidity) of their blood. When the CO2 is then released into the surrounding water through the gills, it causes the pond water to become acidic (lower in pH). In a pond with many plants (including algae), this can result in a significantly lower pH at the end of the night than at the end of the day. At night, it's not just the fish that release CO2, but also the plants (since there's no sunlight). During the day, the plants absorb CO2 under the influence of sunlight, and the CO2 level drops, causing the pH to rise. Pond fish (particularly koi) find excessive pH fluctuations unpleasant. This causes them stress and even health problems. If you measure a difference in pH of more than 0.3 pH points at sunrise and sunset, it's wise to aerate at night and not during the day. This aeration at night “blows” some of the excess CO2 out of the pond.

CO2 and plants

The text above shows that CO2 is essential for plants to produce nutrients (glucose). Underwater plants thrive at CO2 levels between 15 and 30 milligrams per liter. If you have a natural pond with many underwater plants, it's therefore important to aim for these CO2 levels. Many ponds suffer from a CO2 deficiency, which causes plants to become slimy. In a later article, I will demonstrate how you can influence CO2 levels and that there is a direct link between CO2, pH, and KH.