What is Carbonate Hardness (kH)

Carbonate Hardness (kH) can be important for fish that prefers either acidic (eg, Discus) or alkaline (eg, African Cichlids such as Tropheus) whereas it’s not as important for planted aquarium as for the fish.

First of all, when we talk about kh test in the aquarium, it measures alkalinity, that is all titratable bases, and hence does not measure carbonate hardness ONLY. Hence, we will only talk about this topic briefly to have a broad understanding of water chemistry relating to hardness.

Relationship of kH and pH

pH is unlikely to drop when kH is HIGH

pH is likely to drop when kH is LOW

Tropical fish, crustaceans and aquatic plants are sensitive to pH fluctuation. Rapid fluctuation is very stressful to them and if severe it will lead to death.

In case the pH needs to be particular range, it should be changed gradually to avoid severe stress.

What causes kH to change

Factors to lower kH

soil
Nitrification by bacteria

Soil adsorb the calcium and magnesium which leads to reducing the concentration of kH. This then leads to lowering the pH.

The amount of bicarbonate will reduce gradually due to nitrification process by bacteria.

Factors to raise kH

Water change

kH will increase to the measurement of tap water if your aquarium is set in acidic range.