Many atmospheric gases dissolve in water to some extent and carbon dioxide is one of the most important. It dissolves readily to form carbonic acid, giving a weakly acidic solution.
This chemical reaction has important consequences, not only for the eventual chemical content of water, but also for other chemical and biological reactions that occur in the aquatic environment. The naturally acidic nature of rain allows it to bring other, less reactive, substances into solution. (The pH of naturally formed rainwater is about 5.6 but, as we shall see, local atmospheric and landscape variations can alter it significantly before it gets anywhere near our fish ponds.)
The affinity between carbon dioxide and water has many far-reaching effects on water quality, playing a major role in plant and animal respiration and pH buffering.
Carbon dioxide can exist independently in the water from oxygen, so the oxygen levels might be perfectly high and the CO2 can also be high. In air, the increase in one gas necessarily decreases the partial pressure of the others.
Carbon dioxide is not usually a problem in ponds except when ponds are heavily planted. At night, plants respire and release CO2 while using oxygen. So CO2 levels may climb even if oxygen levels also remain high.
By Frank Prince Iles and Erik Johnson DVM