Dissolved gases

The gases dissolved in water include all the gases present in the atmosphere. Your aquarium water even has molecular traces of argon, the "rare" gas that's not that rare— it's much more prevalent in the atmosphere than carbon dioxide (0.94% versus 0.035%) and even a little more soluble. Argon is approximately as soluble in water as oxygen, 2.5 times more soluble than nitrogen, but it is so inert (argon signifies "the lazy one") that it doesn't participate in any of the aquarium's "cycles".
 
"Water is the ubiquitous solvent." Water never remains in its pure molecular form, neither in nature nor in an aquarium— not even a raindrop during its descent. That raindrop absorbs some nitrogen oxides and carbon dioxide before it even hits the earth, and they dissociate into nitric and carbonic acids, even in unpolluted rain. Natural rainfall has a pH of 5.6, which represents the pH of distilled water in equilibrium with the carbon dioxide of the atmosphere. Acid rain can have a pH as low as 3.0, about as acidic as Coca-Cola. But even normal rain in my area around New York regularly has a pH as low as 4.1 to 4.4.

The amount of each gas dissolved in the water represents just a small fraction of what is in the atmosphere. Each atmospheric gas reaches its characteristic equilibrium in water, which depends on the solubility of that gas, on the temperature of the water and on other factors. You know that warmer water is lower in oxygen. Well, it's no different for other gases. Carbon dioxide is a major exception among the dissolved atmospheric gases, because some of it takes the form of dissolved carbonates that form the natural pH buffer.
 
All the atmospheric gases are kept in equilibrium in natural water by the process of diffusion at the surface, where water movement constantly brings new supplies of water into contact with the atmosphere. At any surface, even the surface of a bubble, gases are diffusing inwards into water that is depleted or diffusing out into the atmosphere from water that is proportionately supersaturated. The water surface acts a little bit like a semi-permeable membrane. The diffusion process isn't instantaneous; it can take several hours for the CO₂/O₂ balance to readjust after a densely-planted aquarium has been exposed to several hours of intense light, and the water has become saturated with oxygen and depleted of carbon dioxide. (Stop and think a moment: what effect would this have on the pH?)
 
One misconception we could eliminate right away: it's that carbon dioxide is "exchanged" for oxygen at the water's surface. Nothing so magical can transpire. Oxygen doesn't "drive out" carbon dioxide. What happens is that each gas in its dissolved and atmospheric phases comes into equilibrium through diffusion at the surface. But in an unplanted aquarium with many fish, carbon dioxide levels are likely to be higher than the natural equilibrium, while oxygen is being used up. So water mixing and turbulence at the surface will tend to raise oxygen levels and outgas CO₂. But the two gases aren't directly exchanged.
 
The pages that follow within this folder treat the oxygen, nitrogen, carbon dioxide and other atmospheric gases dissolved in freshwater.