What Is KH And Carbonate Hardness?

KH is the measurement of pH’s ability to stay stable for extended periods. Also known as the “buffering capacity,” alkalinity, or carbonate hardness. When the KH is higher, the pH is harder to change, and the lower the KH, the opposite is true. The pH of the water will naturally rise and fall subtly over time due to outside factors that influence it; like oxygen and Co2, and the nitrogen cycle of your tank. However, significant fluctuations in pH are stressful to fish. It’s best if these parameters (pH, KH, GH, etc.) stay stable, even if they’re less than ideal.

Most of the water’s buffering capacity is thanks carbonates and bicarbonates, hence the term “carbonate hardness.” When you change your water regularly, these carbonates are replenished. There are fresh carbonates and bicarbonates in your tap that keep the buffering capacity up. When we neglect our water changes we use up these compounds and the pH begins to dip. Once enough carbonates are used up, pH can swing wildly due to other factors in the tank, causing osmotic (and general) stress in your fish.

Adding to the confusion, your buffering capacity is also called the alkalinity of your water. Note, this is not the same thing as having alkaline (or basic) water. It can also be called the “acid-neutralizing capacity.”

And this is where the pH rant comes in: carbonate hardness directly affects pH in most water supplies – unless it’s RO water. Because pH is a measurement of acid to base ions and KH is the ability to absorb and bind the acids – making the water more base than acid in most cases. So, generally, high KH comes with high pH, and the converse is often true.

KH is measured in degrees and is noted like this: dKH (degrees of carbonate hardness.) As a general rule, 4dKH is usually the lowest recommended level of KH in any aquarium. The threshold for freshwater is generally recommended to be no higher than 20dKH for African species who like high pH and alkaline water.