Carbonate hardness - alkalinity

The term carbonate hardness is a historical parameter which makes a statement about the buffer capacity (alkalinity, acid binding capacity, acid capacity down to pH 4.3). A buffer is able to keep the pH value more or less constant when acid or alkali is added. Since in aquaristics the main buffer consists of carbonic acid, hydrogen carbonate and carbonate, the term carbonate hardness to some extent represents the acid binding capacity or alkalinity. Carbonate hardness is dissolved inorganic carbon (DIC) in water, along with carbon dioxide and carbonic acid.

In marine aquaristics, calcifying organisms (corals, mussels, calcareous algae, etc.) consume the carbonate buffer together with calcium. Thus, as the carbonate hardness decreases, the water loses its ability to maintain a stable pH. Therefore, carbonate hardness is one of the most important factors in marine aquaristics.

The conversion factor from carbonate hardness (°dH) to alkalinity (mmol/l or mM) is: 0.357. From alkalinity to carbonate hardness: 2.8. Attention! There is also French hardness (°fH) or English hardness (°eH) with other factors.

There are different methods to increase or decrease the carbonate hardness. Make sure that any change should always be done slowly. More info: see links: Checklist-KH-pH.

A dosage of hardness builders and/or the use of a lime reactor should be done during the day, because the carbonate hardness is consumed more during the day.

Optimal KH values:
SPS aquarium with very low phosphate concentrations: 6.5...7.5 °dKH (2.3...2.7 mmol/l).
SPS aquarium with higher phosphate concentrations: 7...9°dKH (2,5...3,2 mmol/l)
Soft corals, fish: 8...10°dKH (2,9...3,6 mmol/l)