The hardness of water refers to the combination of substances based on calcium (Ca) and magnesium (Mg) that are contained in it. The main substances, known as salts, are carbonates, bicarbonates and sulfates.

Water with zero hardness does not contain any of these salts; this is the case with distilled water.

The water in some areas can be particularly hard, mainly due to the presence of limestone (or calcium carbonate). The hardness of water really depends on the land through which it has passed: the

There are kits on the market that offer even the novice aquarist the panoply of tests required to control the majority of the main parameters for water.

more calcium and magnesium the rocks contain, the harder the water. The effects of this can be seen in domestic use: a washing machine, for example, will require more detergent. Above certain limits of hardness (see the table on page 17), water is unfit for human consumption or any other use. Water with a low degree of hardness, i.e. containing few calcium and magnesium salts, is considered soft. Water with a high degree of hardness is classified as hard.


In nature

Life in water, as on land, is not possible without light. Vegetation (microscopic plankton or plants) absorbs it with carbon dioxide (CO2) and uses the mineral salts, which act as nutrients. This vegetation serves as food for herbivorous or omnivorous fish, which in their turn provide nutrition for carnivorous fish. From this point, the next link in the chain can be aquatic (dolphin, shark), terrestrial (man), or aerial (bird). When aquatic organisms die, they fall to the bed. Their bodies are degraded by the action of bacteria, the material is recycled into mineral salts, and so the chain comes full circle. (While they are alive, it is their excreta that are recycled.)


Peat derives from the decomposition of vegetation in an acid environment lacking in oxygen. This process, which lasts several centuries, gives rise to a peat bog from which compact, fibrous peat can be extracted.

It endows water with both a yellow amber color and acidity, which gives it slightly antiseptic properties. This means that some diseases are less common in acid water. The use of horticultural peat, which often has been enriched with various products, must be avoided in favor of the peat for aquarium use that is commercially available. Boil it for around 15 minutes before use.

Hemigrammus erythrozonus.

In the Amazon region of South America, the color of the water ranges from amber yellow to brown, due to the leaves and branches floating in it. In an aquarium, peat can be used in the filtering equipment to reproduce the characteristics of this type of water (low hardness, pH under 7, coloring).

Pterophyllum scalare. T

The hardness of water is expressed in German degrees (°GH or °DH), not to be confused with Celsius degrees (°C) for temperature: 1°GH is equivalent to 17.9 mg Ca/liter, or 17.9 parts per million (ppm). The term most often used to classify hardness is general hardness (GH),

Calcium is a very important element in aquatic life. Vegetation contains only a little of it, but animals have a great deal more: calcium plays a role in the composition of the skeletons of fish, the carapaces of crustaceans, and the shells of mollusks (in the latter two cases, in the form of caiclum carbonate, CaCO^). Calcium and magnesium are more abundant in hard water.

although total hardness (TH) can also be used.

There are three main categories of water in fishkeeping:

- soft water, which is generally acid, at 3°GH or 50 ppm;

- medium water, which is neutral or slightly alkaline, at 6°GH or 100 ppm;

- hard water, which is highly alkaline, at 12°GH or 200 ppm.

We will go on to discover that some fish families can adapt only to certain types of water.

Measuring GH

A colored indicator is used: the number of drops needed to obtain a change in color indicates the degree of hardness. It should be noted that the degrees of hardness used in analysis kits may vary according to the country in which it was manufactured; in some cases French degrees are used. These can be converted as follows:

1°Fr=0.56°GH • How can the degree used by a manufacturer in a product be identified? To confuse matters further, you may also come across °Clark in older books on fish-keeping. The old-fashioned Clark system for hardness was somewhat laborious, being based on measurement of the foam created by a soap solution, and has now become obsolete. If you have any doubts about the units used by the manufacturer of an analysis kit, just measure a GH you already know, such as that of bottled water (see page 23).

The relationship between GH and CH

We have already seen that significant changes in the pH are prejudicial to aquatic-life, especially if they occur too abruptly. To

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