Seachem Aquavitro Carbonate 350ml bottle 

Carbonate hardness, (KH), is a measure of the bicarbonate & carbonate content of the water. Carbonate hardness contributes to pH stability and can also act as a CO₂ backup reserve if CO₂ levels fall too low.

carbonate™ is derived from potassium bicarbonate. It increases both carbonate hardness and potassium. The concentration of carbonate™ is 2000 meq/L (5600 dKH). In addition, it contains 78,300 mg/L potassium.