Alkalinity - what is that?

Alkalinity is used to describe the buffer capacity of your aquarium water. It is the amount of an acid or alkali that you can put into your tank without changing the pH-value.

The official unit of alkalinity is mmol/l. However, the unit °dH (German hardness) has become common in the reef hobby. Most commercially available tests for determining alkalinity in saltwater aquariums use the unit °dH. In the ocean the average alkalinity is about 6.5 °dH.

In seawater the buffer capacity is mainly determined by carbonates and hydrogen carbonates. Boron, silicate, phosphate and magnesium also act as buffers to a small extent. From the measurement of alkalinity you can therefore draw conclusions about the carbonate concentration in your tank.

Why is it important to determine and adjust the alkalinity?

In saltwater aquariums the carbonate concentration plays an important role in the calcification of corals. Their skeleton consists mainly of calcium carbonate. During the calcification process, one carbonate ion per calcium ion is consumed. This leads to a decrease in alkalinity in your tank.

Other processes that lead to a decrease in alkalinity include the assimilation of ammonium and the conversion of ammonium to nitrate during nitrification.

In contrast, an increase in alkalinity is achieved by the assimilation of nitrate, the decomposition of organic compounds, the fixation of molecular nitrogen, the reduction of sulphate and the conversion of nitrate into molecular nitrogen during denitrification.

The alkalinity and the pH-value in your tank are closely connected. The higher the alkalinity, the higher the pH-value and the smaller the pH-value fluctuations.

Therefore, you should always make sure that the alkalinity in your aquarium is in the range of 7.0 to 9.0 °dH.