Why are carbonate compounds added to aquarium water?

In an aquarium, carbonate compounds are primarily added to maintain an optimal level of acidity in the water.1

In closed or recirculating aquatic systems, the accumulation of metabolic wastes can lead to an increase in water acidity. Because many aquatic organisms require a stable pH to function properly, maintaining this balance is a vital aspect of aquarium maintenance. Carbonate compounds act as a buffer, which is a substance that resists changes in acidity by neutralizing hydrogen ions. By adding these compounds, particularly when water renewal is infrequent, aquarists can prevent the water from becoming too acidic and ensure a more stable environment for the organisms within the enclosure.123