Hardness of water caused by carbonate and bicarbonate by-products of calcium and magnesium.Carbonate hardness is a measure of the amount of calcium and magnesium present in water. As it turns out, these elements are vital to the health and well-being of certain fish – including trout, koi and goldfish – because of their ability to neutralize acids in water and stabilize healthy pH levels. The carbonate hardness is the stabilizer for the pH. With low carbonate hardness the pH is unstable, and can change rapidly again and again. To increase the hardness, place stone or gravel containing calcium (e. g. coral sand or marble stone) in the water. Products which contain carbonates, or dissolved baking soda, can also be added to the water to increase carbonate hardness.