KH Alkalinity Test - Carbonate Hardness. Alkalinity is essentially a means of quantifying the stability of pH in a solution, such as aquarium water. It is a measure of the concentration of various negatively charged ionic compounds (anions) such as carbonates, bicarbonates, borate and hydroxides in the aquarium water. If left unchecked, the alkalinity drops below a critical level and pH begins to fluctuate more wildly than is normally suggested (a daily pH shift of +/-0.2 is normal in marine aquaria), causing stress to livestock. In a marine aquarium, the alkalinity should be between 7° and 10° dKH (German carbonate hrdns).  In a fresh water aquarium it should never drop below 3°dKH.