Aqua Medic Hydrocarbonate 5 liter bucket. Medium grain. Is a pure and natural product. It is used in calcium reactors, in filters to increase the hardness of water, and as a filter medium or substrate material in marine or Malawi - Tanganyika aquaria. A natural calcium carbonic acid equilibrium is created in all aquariums with an alkaline pH value and in a calcium reactor supplied with CO2, the Hydro Carbonate is completely dissolved to calcium bicarbonate, becoming available for assimilation by hard and many soft corals, sponges, echinoderms, etc.