The hardness of water is the concentration of “multivalent metals” dissolved in water, usually expressed in dH. It mainly includes multivalent metals such as calcium ions or magnesium ions. In other words, more than 80% of metals are carbonate components. The sulfates and chlorides are usually their residues. The greater the hardness of the water, the greater the amount of substances required for a certain volume to achieve the same purification effect.

The seawater hardness in natural seas is 7°dH -9°dH, which belongs to the category of hard water. In the aquarium, what really stabilizes the pH is the hardness of the carbonate. After a period of operation, the aquarium has established a buffer system for the impact of carbonate hardness on pH. Such as carbon dioxide decomposed by the corpses of sea creatures, the fusion of light, the changes of trace elements in the water, and so on. Through buffering, after a relatively short time, it will automatically return to the original normal value.