CO2 dissolves in water and exists in the form of H ions and Hco3. Symbiotic single-cell zooxanthellae on invertebrates such as corals in seawater require a large amount of C02 to grow. When there is insufficient C02 in the water, Ca ions and Hco3 in the water will combine to form white water-insoluble calcium carbonate and C02 gas. , so that the number of H ions in seawater continues to increase, which is manifested in the continued decline of the PH value in seawater.

The number of calcium ions in seawater continues to decrease, showing that the hardness of seawater is decreasing. When CO2 is added to seawater, CO2 can dissolve white insoluble calcium carbonate and continue to dissolve it in water. In this way, the number of Ca ions and H ions in seawater continues to increase, which is manifested in the hardness and PH value of seawater rising. This is the buffering system of the seawater in the aquarium. As long as the hardness of seawater is stable at 7′-9’dH, the pH value of seawater will be correspondingly stable between 8.0-8.5.

Carbon dioxide and sea water.