The carbonate is the most important buffering system in the aquarium. If sufficient carbonate hardness (alkalinity) is present, addition of acid or base will not change the pH of the aquarium water significantly - the pH remains almost constant. Acid is formed for example by nitrification and sulfur-based denitrification or is released by CO2 driven calcium reactors. If the acid production is not buffered, the pH quickly falls into a poor or even life-threatening range for aquarium organisms.
The carbonate hardness in freshwater community tank should be about 2-4° KH (0.7-1,4 mmol/l). In pure breeding tanks the water should be very soft (<1°KH). Fishes from lake Malawi and Tanganjika feel well at 5-10° KH (1.8-3.6 mmol/l).
In sea water aquarium alkalinity should not be less than 7-8° KH (2.5-2.9 mmol/l) - at KH 8-10° KH (2.9-3.6 mmol/l) you achieve better results.
Our recommendation is a maximum change of the alkalinity of 1-2° KH (0.4-0.7 mmol/l) week.
Test the hardness value (alkalinity) regularly.
The monitoring of alkalinity is as important as the temperature and salinity!