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Chemistry and the Aquarium

Alkalinity2 for our purposes here is comprised of bicarbonate and carbonate. The vast majority of alkalinity depletion in most tanks also comes about by the precipitation of calcium carbonate, as described above. In this process, as alkalinity is depleted by 1 meq/L, calcium will be depleted by 20 ppm. There are some other processes that can lead to alkalinity depletion, including partial cycling of nitrogen (from organic compounds to nitrate and no further) and the incorporation of magnesium into calcium carbonate, but these are generally much less important than calcification.
Consequently, alkalinity depletion in most tanks (especially in short time frames) is tightly coupled to calcium depletion, and if one supplements calcium and alkalinity in proportions equal to those that they are being removed, then it is MUCH less likely that calcium and alkalinity will become imbalanced4 and thereby trickier to correct. That is, the only problems that you will encounter are those in zones 1 and 2 (not enough or too much of these additives). Using a balanced scheme, you should not ever end up in zones 3 and 4, where you have substantial imbalances between calcium and alkalinity.