What is brackish water? It is surprising how many people believe that the term brackish means dirty. If you look it up in the dictionary, you will find that in truth, it means ‘saltish’ or ‘rather salt’. This is a fair description of what we know as brackish although, if you want to be pedantic about it, even fresh water contains salts.

We are mainly concerned with salts contained in sea water, primarily sodium chloride. As for the aquarium, we can take brackish water to mean that which is part way between fresh water and marine or sea water. To put a value on it, a specific gravity of around 1.005 is suitable with anything between 1.002 and 1.008 being quite acceptable.

The specific gravity is simply a measure of the density of water; the more salt added, the greater the density of the water. Pure distilled fresh water has a specific gravity of 1.000; sea water has a specific gravity of around 1.020 to 1.024 although this can vary around the world, with temperature and with the depth of water. To measure this value for your aquarium water you require an instrument called a hydrometer. These are readily available in most good aquatic retailers but one major problem exists; namely that manufacturers seem to produce them almost exclusively for the marine fish keeper. Because of this, they are only calibrated for values much higher than those required for the average brackish water aquarium - usually around 1.018 to 1.030. ...

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