Alkalinity helps the water in your aquarium protect itself against drops in the pH levels. An alkalinity test measures the levels of several elements such as bicarbonates, carbonates and other crucial elements. Although the term Alkalinity and Carbonate hardness are used interchangeably, they do not mean the same thing. Carbonate hardness only measures the waters bicarbonates and carbonates. Alkalinity is a measurement of those elements and several others. Saltwater in the ocean has a carbonate hardness of 6 - 7 dKH and an alkalinity of about 2.5 meq/L.