Calcium carbonate

What is it?
Calcium carbonate is a chemical compound. It is commonly used medically as a calcium supplement or as an antacid. It is also a common substance found as rock (like marble) in all parts of the world and so is often found in nature dissolved in water which flows over these rocks. It is therefore the principle cause of hard water.

is the main building block of seashells and the shell of snails and all shrimps. So Coral sand is mostly.

Types available
In order of dissolving rate. Fastest first.

Limestone Powder, Indigestion tablets, Coral Sand, Coral Gravel, Cuttle Bone, Sea Shells.

Raising GH
Two teaspoons (about 4 grams) of calcium carbonate per of water will increase both General Hardness (GH) and Carbonate Hardness (KH) by approximately 1 degree.

Links

 * Wikipedia
 * Water hardness
 * Calcium carbonate raising KH