The hardness of seawater is determined by the amount of calcium and magnesium ions contained in the water. The hardness of natural seawater is mostly maintained at 7-9dH. In the aquarium, the hardness of seawater should be controlled at 7-9dH. The hardness of seawater changes frequently. When the hardness of seawater in the aquarium drops to 5-6dH, it means that the amount of CO2 in the seawater is decreasing, and the amount of calcium ions in the seawater is decreasing.

The hardness of seawater can be measured using special chemicals. In the aquarium, the stability of the PH value of seawater depends to a large extent on the stability of the hardness of the seawater. That is to say, there is a dynamic buffering balance system between the seawater, PH value and hardness of the seawater in the aquarium.

The hardness of seawater is determined by the amount of calcium and magnesium ions contained in the water.