Hardness is a measure of dissolved mineral salts, mainly calcium and magnesium carbonates, in water. The more dissolved salts, the harder the water. Water dissolves salts and minerals as it percolates through the earth's strata and flows over rocks etc. during the Water Cycle.

The terrain over which water flows greatly affects hardness. Some rocks such as slate and granite contain little or no soluble salts and therefore have negligible influence on hardness. Others such as limestone, found in the African Lakes, have a greater effect. Hardness has a direct affect on the cellular functions of fish, plants and microorganisms and although fish from hard water areas do not seem to suffer from softer than natural conditions the reverse does not apply. Many plants in particular perish in hard water and it is thus necessary to choose the aquarium decor and substrate accordingly to create a hardness indicative of the fishes natural habitat, especially if soft water is a requirement. To test decor or gravel for calcium and magnesium carbonates, add a sample to vinegar (which contains acetic acid). If is fizzes then it contains calcium and / or magnesium carbonates and one should consider the hardening effect it will have on the water.

Three types of hardness are of interest to the fish keeper. Temporary hardness, caused by calcium and magnesium bicarbonates, permanent hardness caused by calcium sulphate, and total hardness (GH) - a combination of temporary and permanent hardness.

Temporary hardness is sometimes referred to as carbonate hardness (KH) or acid binding capacity (ABC), and is easily removed by boiling. Boiling takes out bicarbonates of calcium and magnesium, which are soluble in water, by precipitation. It is this which in hard water areas creates a white scale in the kettle.

The fraction of total hardness which cannot be removed by boiling is called permanent hardness. Permanent hardness can therefore be determined by subtracting the figure for temporary hardness from the figure of total hardness.

Carbonate hardness (KH) may often be greater than total hardness (GH) because of the presence of sodium and potassium in addition to calcium and magnesium. These elements do not increase hardness but occur along with bicarbonate anions to increase the quantity of bicarbonates.

Hardness is expressed in degrees but the units differ form country to country and test kit measurements vary depending on whether total hardness or carbonate hardness is being determined. Throughout this site, we shall refer to degrees hardness based on milligrams per litre (mg/l) of calcium carbonate (CaCO3). This is equivalent to parts per million (ppm) calcium carbonate. You can use the conversion calculators to convert between unfamiliar units and mg/l CaCO3.

The majority of fish prefer hardness below 200 ppm but Lake Malawi and Lake Tanganyika cichlids may prefer higher values of up to 400 ppm.

To simplify the units further, the following common terms are generally used to describe hardness.

As well as occurring naturally, hardness in the aquarium can be influenced by the reaction of small amounts of carbonic acid in the water (from decaying material) with calcium and magnesium carbonates in the decor to produce calcium and magnesium bicarbonates. The presence of these bicarbonates acts as a reservoir or buffer for carbon dioxide in the aquarium and helps to stabilize pH. It is for this reason that soft water is usually acidic in nature and hard water is usually alkaline. Have a look at the relationship between Hardness, pH and CO2 here.

To soften water:

• Boil it, to remove temporary hardness.
• Filter through peat.
• Grow duck weed or Egeria densa. These plants absorb calcium during their rapid growth.
• Dilute with clean or distilled water.
• Use an ion exchange resin.
• Use water processed by a reverse osmosis unit.

Distilled and reverse osmosis water should never be used "neat" as both processes remove free oxygen. Not because they remove minerals as often stated. To rectify this problem, aerate the water vigorously before use.

To harden water:

• Use lime rich decor such as tufa rock or a coral sand substrate.
• Filter through marble chips or other calciferous material.

Remember to consider the resulting pH change when hardening or softening water.