Carbon dioxide content in stagnant bodies of water is determined in the same way as oxygen content is determined, mainly through photosynthesis and respiration, however with a changed algebraic sign: Photosynthesis consumes CO2, which is released through respiration. Additional input and output of carbon dioxide occurs via the gas exchange with the air in proportion to increasing water movement.
Carbon dioxide dissolves much more easily in water than does oxygen, because together with water it forms carbonic acid, which is easily soluble.
|Carbon dioxide||+||Water||→||Carbonic acid|
Carbon dioxide dissolves solid calcium carbonate and forms dissolved calcium hydrogen carbonate.
|+||Carbonic acid||↔||Calcium hydrogen carbonate|
(Soluble calcium carbonate)
Water that contains carbonic acid is also capable of dissolving lime, (scientists refer to it as calcium carbonate), through a chemical/physical process. The two arrows indicate that the reaction can progress in both directions. If carbon dioxide is withdrawn from the water, insoluble lime and carbon dioxide form until the balance is restored.
Insoluble lime is recognised as a white precipitation that is familiar as limescale. Since water when heated loses the ability to store lime in dissolved form, lime precipitates as limescale.
The lime content of the water influences the pH value, i.e. lime content determines whether the water will have a neutral, acidic, or alkaline reaction. The greater the concentration of dissolved lime, the more carbon dioxide in the form of carbonate of lime - referred to as calcium carbonate by specialists, is bound and the less free carbonic acid there is in the water. Consequently the pH value tends to move in the slightly alkaline range around pH 8. On the other hand the free carbonic acid, as its name indicates, causes a lowering of the pH values in the direction of the acid range.
In the field of fishkeeping frequently CO² discharge due to aeration is mentioned. This phenomenon occurs in the garden pond only to a very slight degree due to the greater water surface. However as opposed to the situation with tropical ornamental fish you should strive for a pH value from 7 to 8.5. Thus a slight increase is desirable. At pH values around 8 undersaturation occurs, which causes the water to automatically be enriched with the CO² contained in the air. Thus the aeration is harmless.