From The Aquarium Wiki
What is it?
Coral sand is a mostly white medium fine crushed coral and shells (though sometimes Aragonite is added). The shells of freshwater or marine invertebrates is made of [Limestone], the mineral calcite (calcium carbonate CaCO3).
What is it used for?
- To make a substrate for an aquarium tank.
- To add calcium to the water to increase the GH levels.
- To add carbonate to the water to increase the KH levels. Typically to buffer soft water and to raise pH.
How much to use?
Approximately 1 handful per 10 Litres (2.6 US G.) spread over the substrate.