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  • Writer's pictureStewart Kessel

Water Chemistry: Hardness KH & GH

Updated: Jan 8

What is Water Hardness - Why is it Important in an Aquarium?

In everyday life, we come across hardness of our water, whether it is how easy it is to lather soap, or how quickly the kettle furs up with limescale. Depending on where you live, you may be in a hard-water or a soft-water area.

Water hardness refers to the amount of dissolved mineral content of a given water solution. In soft water areas, the dissolved mineral content is relatively low, and for example, soaps will lather quickly. In areas with high mineral content and hence hard water, kettles will typically very quickly fur up with limescale.

Sometimes the terms Temporary Hardness and Permanent Hardness are used. By way of an example, when boiling a kettle, this causes carbonates in the water to settle out, usually as calcium carbonate (limescale) – this is no longer dissolved in the water and therefore referred to as Temporary Hardness and the mineral salts that remain are Permanent Hardness. Equally, if you live in a hard-water area, you may become used to seeing a white scale layer of the limescale on your aquarium glass at the surface where the water evaporates.

Water hardness is one of the most critical parameters of a freshwater tank. Different species of fish, invertebrates and plants prefer specific ranges of water hardness.

In the aquarium hobby we focus primarily on carbonate, calcium and magnesium contents, and to measure total hardness, we look at two parameters:

Carbonate Hardness (KH): A measure of the carbonate and bicarbonate minerals in the water. Sometimes referred to as total alkalinity, it is also a measure of the aquarium water buffering capacity, a critical factor in pH stability. Learn more in our article on pH and buffering capacity.

Carbonate Hardness value is expressed in degrees KH, where one °d KH equals 17.85 ppm or 17.85 mg/l calcium carbonate (CaCO3).

Maintaining a minimum KH level of at least 4°d KH at all times is advised, with a KH of between 6 and 8°d KH more desirable - this will help in maintaining the pH stability of the aquarium.

General Hardness (GH): A measure of the total amount of all minerals dissolved in the water, i.e. magnesium and calcium dissolved salts and other trace minerals.

With a General Hardness test kit, the total weight of all ions is typically simplified and expressed as the equivalent weight of calcium carbonate (CaCO3), where 1 °dH (German hardness) is equal to 17.85 ppm or 17.85 mg/l equivalent weight of calcium carbonate (CaCO3).

The correct level of GH will depend on the livestock. For soft water species, a typical GH level is around 3 to 6°dH, while for most hard water species 10 to 15°dH is a more appropriate range.

German hardness °dH or mg/l (ppm) CaCO3 are the most common measurements to describe hardness in the aquarium industry but other less common scales exist:

°Clark Degrees (equivalent to 14.25 mg/l CaCO3), sometimes used in the UK

°FH French Hardness (equivalent to 10 mg/l CaCO3)

°Hardness (equivalent to 1 mg/l CaCO3).

Water Hardness Descriptions: The table below shows water hardness descriptions by value as used by most water authorities in the UK:

Both KH and GH are depleted continuously through the natural process in the aquarium - this can have a detrimental effect on aquarium stability and health of the livestock. Therefore, routine monitoring and appropriate aquarium maintenance are advisable. Read our article on Old Tank Syndrome to learn more.

Note that the reverse can also be observed, with rises in GH and KH typically caused by unsuitable aquarium hardscape materials. For example, many rocks can affect hardness by leaching soluble materials, such as limescale, into the aquarium water. It is advisable to test all such hardscape materials before using them in the aquarium.

Author: Stewart Kessel CChem, MRSC


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