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Alkalinity is a measure of the "pH buffering capacity" of aquarium water. The main buffering components are carbonates and bicarbonates.
3 - 4 meq/l or roughly 8 - 12 dKH (1meq/l = 2.8 dKH)
Note: values of up to 16dKH are possible, and should not have negative effect.
To understand alkalinity, we first have to explore some properties of acids. Note: the following description contains gross simplifications. Not all acids are born equal. Some are inherently stronger than others. Industrial acids such as hydrofluoric acid and sulphuric acid are like alien-blood... they'll eat a hole through concrete! Other acids, such as the types we add to food (citric acid and acetic acid - vinegar) we can consume quite happily, with no ill affect. For any given concentration, a strong acid will have much more affect on the pH than a weak acid.
Fine... so what? If we could somehow turn the strong-ish acids that are produced in the aquarium into weak acids, they would have less influence on the pH, stabilising our system. Cool, but how is that done?
The acidity of a substance is caused by free hydrogen ions (H+). The predominant buffer contributing to the alkalinity in our aquaria is the carbonate ion (CO3--). The carbonate reacts with the hydrogen ions, forming carbonic acid, which is a weak acid. Ta da... problem solved! Even better, the carbonic acid can then "break down" into carbon dioxide and water. We have substituted a great evil for a lesser one, stabilising our system.
Interactions: pH and alkalinity
From the above discussion, it is obvious that there is a relationship between pH and alkalinity. A low alkalinity means that there are less carbonate ions to bind to acidic hydrogen ions, allowing wild fluctuations of pH, destabilising the micro-environment. This is particularly evident as large day/night pH swings.
High pH (above 8.6) can cause the spontaneous precipitation of calcium carbonate. This drags carbonate ions out of solution, lowering the alkalinity. Ironically, this in turn allows sudden drops in pH.
Interactions: calcium and alkalinity
Caution should be taken when increasing calcium levels in a reef aquarium. The predominant influence of calcium on carbonate levels and hence alkalinity is again the accidental precipitation of calcium carbonate, which can be caused by any of the following: