The carbonate hardness (abbreviation KH) is one of the most important parameters in a marine aquarium. The aquarist should regularly measure this value, because in a well-functioning biotope marine, the need for carbonates is enormous. The carbonate hardness can decrease by 5-10 ° in a matter of only days.

The hard corals like Acropora spp. need not only calcium but also carbonates.

The hard corals like Acropora spp. need not only calcium but also carbonates.

What is the carbonate hardness?

The carbonate hardness in the world of marine aquariology, defines the ability to stabilize the pH value of the water. All common tests used in aquariums measure only the buffer substance present. This common definition in our environment does not have much to do with the carbonate hardness as it is conceived in other areas. Unfortunately this fact is described only in a few aquarium books, and the definitions are sometimes expressed in a contrasting manner. The best definitions for “carbonate hardness” are: “ability to bind acids,” acid capacity pH 4.3 “or” buffering capacity “. But for easier understanding, although the term is not quite correct, we will continue to use the term “carbonate hardness” in the article. We will also use, for simplicity, also the old definition of degrees: ° dKH = German degrees of carbonate hardness. A buffer system consists of at least two components, a weak acid and a corresponding salt. What is essential in a buffer system is its effect on neutralizing acids. If a liquid possesses such a characteristic, its pH value will vary and become weaker when an acid is added, than it would in the absence of this capacity. If you add an acidic substance, like the products to metabolize the water of an aquarium, the salt can bind with these neutralizing substances. The “carbonate hardness” in the aquarium consists mainly of the buffer system carbon dioxide-carbonic acid – hydrogen carbonate-carbonate. Systems such as phosphoric acid-phosphate hydrogen phosphate play a limited role.

The higher the value of the carbonates, the less the pH value will be affected by acidic substances. The carbonate hardness stabilizes the pH value of the aquarium 

What is the carbonate hardness in seawater?

In a marine aquarium the carbonate hardness has two basic tasks:

Stabilization of the pH value

As already mentioned, the carbonate hardness is characterized by a buffer system. In a marine aquarium it should never fall below 5 ° dKH. Below this value the fluctuations of pH in the aquarium water, are so huge that the animals can suffer serious consequences. If the value of the carbonate hardness should become around 2 ° dKH, the pH value of a reef, in the course of the day, will fluctuate well outside the normal limits (7.9 to 8.4). For animals, a variation of only one point, for example from pH 8.5 to pH 7.5, signifies a change in the level of acidity in the water by 10 times, with a two point change that changes to 100 times. The acidity causes a lot of damage to the fish gills and also to the delicate tissue of the coral polyps, which in nature are obviously never subjected to such changes. As a precaution the aquarist should never let the value of the carbonates go below 7 °. To maintain a stable condition in the aquarium, it has been proved that a good value of carbonates is around 10 °. Natural sea water, however, often shows values of only 7 ° dKH, but in a limited environment such as that of an aquarium, the acidic substances derived from animal metabolism cause a much higher concentration, which must obviously be corrected. In addition the photosynthetic activity of symbiotic algae affects the pH of the aquarium, because of the limited amount of water, in a much more obvious way than if in the sea. For these reasons, the carbonate hardness of an aquarium should always be higher than that measured in nature.

Supply of calcium to consumer animals and algae

Many marine animals need, for the construction of their skeletons, dissolved calcium, especially hard corals and clams, which incorporate large amounts of this substance. Even the calcareous algae, such as the reddish Mesophyllum species that develop as a crust, or the green ones from the species Halimeda, extract a lot of calcium from the water. This important substance is characterized by two components: calcium and carbonate. If one, or both components are missing symptoms of deficiency occur. Especially for small polyp stony corals, the lack of calcium causes a rapid block in development.

A Carbonate hardness value of more than 15 ° dKH provides no advantage and should be avoided, because it may indicate precipitation phenomena. The consequence is that some substances, important for the development of the animals, would be lost by binding to each other. 

Texts by Burkhard Ramsch and Beate R. Sellner

Biologists Company AquaCare, Herten, Germany.

Photo: D. Knop

Part 2 will follow