Sea Water

The main difference between fresh water and sea water is that the latter contains a great many salts which give it certain specific characteristics, and these must be understood by any aquarist who wishes to keep marine fish.

A Very high quality sea water is required in marineaquariums, especially if they contain invertebrates, particularly anemonesand corals.


The temperature of tropical sea water varies little over the course of a day, or even a year. Furthermore, marine fish are generally more sensitive to abrupt changes than freshwater fish. The temperature in an aquarium must, therefore, be fairly stable, remaining at around 25-26°C.


The most important salt found in sea water is sodium chloride (NaCl), widely used for domestic and culinary purposes, but there are plenty more. The salinity of water, i.e. the quantity of salts in the water, is expressed in /00 or in g/liter. The mean salinity of the Earth's oceans is around 350/oo, or approximately 35 g salts/liter.

Whatever its salinity, sea water boasts one remarkable property: the proportion of each element is constant. Desalinated water does not therefore contain less of one or more salts, but the combination of salts is present in a lower concentration.

The salinity of sea water varies according to longitude. It is at its highest in open seas in the tropics, it is lower near coasts and after heavy rain, and it is at its lowest near the poles (due to the influence of melting snow).


In marine aquariums, it is not the salinity of water which is measured, but the density (often expressed as specific gravity, S.G.), which can be calculated according to the following formula:

There are no units of measurements. The saltier the water, the higher its density. The density also varies according to temperature (it goes down as the temperature goes up). The table overleaf shows the relationship between salinity and density with respect to temperature, which is relatively constant (25-26°C) J in aquariums. The density, expressed as specific gravity, a value which is easy to use, is all that is required to calculate salinity: it must range between 1.022 and 1.024.


Unlike the general hardness (GH) which is used to describe fresh

Thehydrometer, an indispensable tool in marine aquariums. •


Density is measured with a hydrometer, whose buoyancy increases as the water gets saltier. In the aquarium trade, most hydrometers also include a thermometer. The specific gravity at water level must be read with care; in fact, it is preferable to use the hydrometer outside the aquarium, as the movement of the water makes it difficult to read. In this case, decant the water into a test tube or a transparent container (a PVC bottle, for example) and float the hydrometer in it. When it stops moving, read the value corresponding to the level of the water (1.023 in the diagram below, and not 1.022). To check whether your hydrometer is working properly, just measure the density of a distilled or very soft water: it must equal 1.000.

wafer level

hydrometer wafer level


Some examples of the salinity and density of sea water

Geographical areas

Cornish Atlantic coast

Western Mediterranean

Atlantic Ocean

Tropical reef zones

Red Sea

Salinity in %













water, carbonate hardness (CH) is used in sea water, where it serves to measure the quantity of calcium and magnesium carbonates and bicarbonates present. This is crucial for maintaining the pH, and for growing corals, which have skeletons made of calcium carbonate.

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