Hardness avoiding confusion

The use of different terms and abbreviations sometimes makes hardness a somewhat confusing concept. The table below presents the expressions most often found in recent aquarium books.

Metric value


Degree value


Term mg/liter; ppm

General hardness (total hardness)

Measures all the calcium and magnesium salts

Measures only the calcium and magnesium carbonates and the bicarbonates.

mg/liter; ppm

Carbonate hardness

The general hardness is one of the main parameters to be taken into account when monitoring the water in an aquarium. The carbonate hardness only measures carbonates and bicarbonates, the latter being in the majority in soft water with a pH less than 8.

A Capeta tetrazona(here the golden variety) prefers soft to medium-hard water, especially for reproduction.

compensate for this, nature has provided a screening device, the CH (carbonate hardness, i.e. the hardness due to calcium and magnesium carbonates and bicarbonates). The higher this is, the less the risk of any major variations in the pH. and vice versa. This phenomenon, known as buffering, can therefore only occur in acid fresh water. There is a relationship between the CH and the general hardness: the closer the CH value comes to the GH value, the more balanced the water. If the CH is less than 75% of the GH, you are likely to encounter a problem, and it is therefore not advisable to use water with these characteristics in an aquarium.

Modifying the hardness of water

Sometimes the water available presents a hardness value inappropriate for its intended use in an aquarium. In most cases, the water will be a little too hard, and so the GH must be brought down for use in a mixed aquarium or a rearing tank. In other, less common cases, the water can be slightly too soft, and so the GH needs to be raised.

• Reducing the GH Water with a low hardness value can be mixed with water that is too hard. There are several alternative sources of water

- spring and well water;

- defrosting water from a refrigerator;

- water from melted snow;

- distilled water, available in bottles;

- some brands of mineral water;

- natural flowing fresh water.

The volume of water that can be obtained, and its price, obviously depend on which of these sources is used. Filling a tank with a capacity of several hundred liters with water of a precise hardness can sometimes be a laborious process. A final piece of advice: avoid using water from a domestic softener, as the calcium salts are replaced by other salts. Osmosed water (see page 17) is an attractive option, but the equipment represents a substantial investment. • Increasing the GH

The water in question can be diluted with harder natural water, generally easier to find than soft water, or put some calcareous rocks in the aquarium, regularly monitoring the GH, or filter the water over oyster shells crushed into tiny pieces. Any modification in the hardness of water is matched by a modification in the pH: increasing the hardness of the water also increases its pH, and vice versa.

Obtaining water with a precise hardness Let us suppose we have two types of water, one hard and one soft, with which to "manufacture" an intermediate water:


GH water A - GH target water = 9-5 = 4. GH target water - GH water B = 5 - 3 = 2. The combination of 4 liters of water B and 2 liters of water A results in 6 liters at 5°GH. Filling a 180 liter tank will require 180 (6 x 30 times this mixture, i.e. 60 liters of water A and 120 liters of water B). Another example with the same water: filling the same tank with water at 7°GH will require 120 liters of water A and 60 liters of water B.

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