Water Hardness: GH and KH

The difference between general hardness (GH) and carbonate hardness (KH), how they affect fish and plants, units, and how to raise or lower them.

What hardness is

Water hardness describes the concentration of dissolved minerals. It is split into two measures relevant to aquariums: general hardness (GH) and carbonate hardness (KH). They describe different ions and are adjusted differently.

General hardness (GH)

GH measures dissolved calcium and magnesium ions, which enter water as it passes through limestone, chalk or gypsum. These minerals are biologically important, particularly for invertebrates such as shrimp and snails, and for plants. Very low GH, alongside low total dissolved solids, can cause problems for shrimp, snails and some plants.

Carbonate hardness (KH)

KH, also called alkalinity, measures carbonate and bicarbonate content. It is the water's acid-neutralising capacity and is what buffers pH against downward drops. KH and GH are independent: water can be high in one and low in the other.

Units

Hardness is expressed in mg/L as calcium carbonate (CaCO3), in degrees (dGH or dKH), or in ppm. As a conversion, 1 degree of hardness equals about 17.85 mg/L CaCO3. By the USGS scale, 0-60 mg/L is soft, 61-120 moderately hard, 121-180 hard, and 181 and above very hard.

Effect on fish and plants

Soft-water fish generally do best in neutral to acidic water, and hard-water fish in neutral to alkaline water, though many commercially bred fish tolerate a wide range. Most aquarium plants prefer low KH (often around 3 dKH and below), while moderate GH supplies the calcium and magnesium plants and invertebrates need.

Raising and lowering

To raise GH, calcium and magnesium are added via mineralising salts or hardening additives. To raise KH, carbonates/bicarbonates are added. To lower hardness, mineral-poor water such as reverse osmosis or rainwater is blended with tap water; boiling can reduce temporary (carbonate) hardness.