Aragonite
Crystalline form of calcium carbonate. It makes up the skeleton of corals, living sand and the substrate of calcium reactors.
Detailed explanation
Aragonite is one of the two crystalline forms of calcium carbonate. It is the material of which coral skeletons and the shells of many marine molluscs are made.
As a substrate in marine aquariums it offers several advantages: it buffers the pH, provides a natural environment for microfauna, and has an ideal grain size (1-2mm).
The most used brands are CaribSea Arag-Alive (moist sand pre-seeded with bacteria) and CaribSea Special Grade.
In calcium reactors, aragonite is dissolved by CO2 to release calcium and carbonates into the water.
Aragonite sand should be kept at 2-3 cm (shallow sand bed) or 10-15 cm (deep sand bed). Intermediate thicknesses (4-8 cm) are problematic.
Aragonite is not used in freshwater aquariums because it would raise the pH and hardness. It is only used in African cichlid aquariums.