Calcium carbonate has a very low solubility in pure water (15 mg/L at 25°C), but in rainwater saturated with carbon dioxide, its solubility increases due to the formation of more soluble calcium bicarbonate. Calcium carbonate is unusual in that its solubility increases as the temperature of the water decreases. The limestone does not dissolve in water but it will form a solution same as that of mud and water. Two separate layers can be seen. If limestone is mixed with a mixture of carbon dioxide and water it will form calcium hydrogen carbonate. Yes. If your rocks are actually limestone, they are the cause for the pH elevation in your aquarium water. Limestone is calcareous and it is for its ability to both harden the water and increase the pH. Carbon dioxide in the air dissolves in rainwater and becomes weakly acidic. This weak “carbonic acid” is able to dissolve limestone as it seeps into cracks and cavities. Over many years, solution of the rock can form spectacular cave systems.

Yes. If your rocks are actually limestone, they are the cause for the pH elevation in your aquarium water. Limestone is calcareous and it is for its ability to both harden the water and increase the pH. Carbon dioxide in the air dissolves in rainwater and becomes weakly acidic. Calcium Carbonate the preceding equations show that the presence of CO2 will increase the solubility of CaCO3 in brine. Increasing CO2 also makes the water more acidic and decreases the pH. Calcium carbonate combines with carbon dioxide-saturated water to produce soluble calcium bicarbonate. Limestone is unique because its mineral grains can be dissolved in nature by a very mild carbonic acid. The weak acid forms from a mixture of water from rain and carbon dioxide from air and the soil. When sulfurous, sulfuric, and nitric acids in polluted air and rain react with the calcite in marble and limestone, the calcite dissolves. In exposed areas of buildings and statues, we see roughened surfaces, removal of material, and loss of carved details.