Calcium Carbonate

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Calcium Carbonate
Common Name(s): Aragonite



Molecular Formula: CaCO3
Used For: Sand bed substrate

Calcium carbonate reactor substrate

Caution: -
Material Safety Data Sheet: Calcium carbonate


Calcium carbonate (molecular formula = CaCO3) is the major component of coral skeletons, which is what forms the reef structure.

There are two crystalline structures that are important for the marine aquarist, Aragonite and Calcite. Aragonite is the form laid down by corals, so is present in coral skeletons (i.e. liverock) and coral sand (which is just broken down coral skeletons). Aragonite can then be transformed into calcite, after being submerged within the Earth's crust and exposed to high pressure and temperatures, forming marble. Calcite is more stable than aragonite and is the crystal form in marble. Since it is more stable, it requires a lower pH to dissolve, for example in a calcium carbonate reactor.

At the pH of a marine aquarium system, ~8.2, calcium carbonate is stable and will not dissolve. The pH has to be lowered below 7.0 for aragonite to be dissolved to any appreciable amount, and a couple of decimal points lower for calcite.


Sand Bed Substrate

Both forms of calcium carbonate are fine to use as the substrate for a sand bed, whether it is a deep or shallow sand bed. Both are inert and if from a clean source wont release anything else back into the water (such as heavy metals).

A common myth is that using aragonite for a sand bed is advantageous as the pH within the sand bed will be low, dissolving some of the substrate and adding calcium to the system. However, due to the small amount that will actually dissolve, volume of the sand bed etc, this will have negligible effect on the calcium concentration within the water. Therefore, another method of calcium addition is required.

Calcium Carbonate Reactor Substrate

Both forms are also fine to use within a calcium carbonate reactor as the substrate being dissolved. However, due to calcite's higher stability, it requires a lower pH to dissolve and therefore more carbon dioxide.


Crushed marble can be sourced from the following companies:

New South Wales

Mini Crete Pty Ltd[1], 17 Chapel Street, Lakemba, NSW 2195, Ph: 02 9740 5722, as of 17/06/2008, Mini Crete charge - $13.50 per 25Kg (variety of grades available)


Gilbert Transport Pty Ltd [2], 20 Thackray Road, Port Melbourne 3207, Ph: 03 9646 2761, as of the 09/01/2009, Gilbert Transport charged the following (minimum order of $50 applies), $18.00 for 25kg of White 000 (0.5-1.2mm), $12.00 for 25kg of White 00 (approx 2mm), & $11.00 for 25kg of White Pool Dust (< 0.5mm). As of 05/07/2009 min. order is going up to $150 and soon you'll need an ABN.