One of the most common complaints of new aquarists is that their aquaria seem to need more alkalinity than their balanced additive system, such as limewater, is supplying. While there are reasons this may actually be the case over the long term (these will be detailed later in this article), frequently these aquarists are seeing a "chemical mirage" rather than a real excess demand for alkalinity.
One of the interesting features of seawater is that it contains a lot more calcium than alkalinity. By this I mean that if all of the calcium in seawater (420 ppm; 10.5 meq/L) were to be precipitated as calcium carbonate, it would consume 21 meq/L of alkalinity (nearly 10 times as much as is present in natural seawater). In a less drastic scenario, let's say that calcium carbonate is formed from aquarium water starting with an alkalinity of 3 meq/L that it is allowed to drop to 2 meq/L (a 33% drop). How much has the calcium declined? It is a surprise to many people to learn that the calcium would drop by only 20 ppm (5%). Consequently, many aquarists observe that their calcium levels are relatively stable (within their ability to reproducibly test it), but alkalinity can vary up and down substantially. This is exactly what would be expected, given that the aquarium already has such a large reservoir of calcium
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