Calcium is the fifth most abundant element in the Earth's crust. It is essential for living organisms, particularly in cell physiology, and is the most common metal in many animals.
Calcium is essential for the normal growth and maintenance of bones, scales and teeth, and calcium requirements must be met throughout life.
Snails and shrimp for example use Calcium to build their shells and Carapace. Without enough Calcium in the water they soon die.
The measure of General Hardness (GH) is the the amount of Calcium (or Magnesium) in the water. So for snails and shrimps they require a GH of at least 6dh.
Other methods are to add an amount of Coral Sand/Gravel to the substrate as this is made of crushed sea shells.