Federation of British Aquatic Societies



Les Pearce, Island Fishkeepers

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 an S.G of 1.000; sea water has an S.G 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.

A good alternative is a plastic hydrometer of the kind used by home brewers. These are quite cheap and you can easily obtain one from any shop that sells home-brewing equipment. They are ideal, being designed to measure in the required range.

Without doubt, the best type of salt to use in an aquarium is a good proprietary brand of synthetic sea salt, obtainable from most aquatic retailers. A cheaper but a far poorer substitute is ordinary cooking salt and, cheapest of all if you happen to live near the sea, is sea water diluted down with fresh water. You should be aware that this latter option carries certain risks and that you can easily transmit disease and pollution to your tank.
This is not an option that I have ever employed, nor would I personally recommend it - and I live right by the sea!

Under no circumstances should you use iodised table salt as this can be very detrimental to fish. The quantity of salt added is very much dependent upon personal preference - what works best for the individual aquarist and on the species of fish he or she intends to accommodate. I have known people who are adamant that one teaspoon of salt per gallon is quite adequate while others have successfully used up to three tablespoons per gallon.

Generally, anything that produces an S.G within the previously defined parameters should produce favourable results.
It has been suggested that when you carry out water changes, it could be beneficial to the fish to vary the salt content slightly with each change.
In nature, this fluctuation will occur with the seasons. At times of heavy rainfall increased quantities of fresh water flowing into estuaries from swollen rivers will dilute the salt content of the water; conversely, hot weather and drought will cause the salt levels to become more concentrated due both to a decrease in fresh water in the rivers and from evaporation.

By far the most common occurrences of brackish water in nature are the estuarine areas where fresh water rivers meet the sea.

Most of the fish that we frequently keep in brackish aquaria are either from these areas or adapted to migrate from the sea into the rivers and vice versa. The most commonly known example of the latter is probably the Salmon, although this fish is not usually kept in aquaria.

This fish is born in the relative safety of a river where it spends the first part of its life. As it grows, it migrates downstream to the sea where it will live and grow to adulthood. It will then return to the river to spawn and the whole cycle starts again. Because of this life cycle the Salmon, and other species of similar behaviour, can readily adapt to a wide range of water conditions, including brackish.

Fish and plants have a tendency to adjust their internal salt levels to those of the surrounding water. Water molecules are small enough to pass through the membrane of cell walls; larger salt molecules are not. This passing of molecules through a membrane is called osmosis. The only way, therefore, for a freshwater fish or plant to maintain body salt levels when placed in brackish or salt water is to expel water, it cannot take on salt. It may sound strange but by expelling body fluid in this way the fish, or plant, can become dehydrated and die.

Because they inhabit tidal or estuarine waters, most fishes commonly kept in brackish conditions prefer a certain degree of water movement.

The high volume of water turned over using power filtration can, therefore, be most beneficial. In order to help maintain hard, alkaline water a substrate containing calcium such as coral sand or gravel, crushed cockle shell or Calcium Plus is advisable. If you wish, you can mix this with ordinary gravel or use it on its own as an alternative to gravel. Because of the flow over the gravel, a sub-gravel filter will obviously improve the effect of this substrate on the water chemistry.

The problem of dehydration caused by osmosis means that the variety of plants that can adapt to brackish water conditions is somewhat limited. Generally, plants with tough, waxy leaves such as Java Fern, Vallisneria and Sagittaria do best. Other factors in some brackish water aquaria can also be detrimental to the cultivation of healthy plants. As mentioned earlier, most brackish water species prefer water movement ... typically, however, plants do not. Many brackish water fish species commonly kept in aquaria are, to a greater or lesser degree, herbivorous and can do untold damage to a well-planted tank.

The Spotted Scat, Scatophagus argus, for example, is an avid eater of plants. This species can grow to ten inches or twenty-five centimetres long under the right conditions and is a very deep-bodied fish. Imagine what such a herbivore could do to your plants!

The range of fish available to the brackish water enthusiast is wide and varied.

Probably the most commonly seen of these are the Mollies, Poecilia sphenops, Poecilia latipinna and Poecilia velifera. (Incidentally, they derive their common name from their old scientific genus name, Mollienisia. This has now been changed to Poecilia but the common name has remained).

These livebearers are cheap to buy and easy to keep, adaptable as they are to a wide range of conditions. They are fairly easy to breed ... the female gives birth to relatively large, well-formed live young. They are selectively bred in a wide range of colours and forms, making them a bright and attractive choice for your aquarium.
Mollies make an ideal introduction to the keeping of brackish water species.

Many families of fish that we are familiar with in the freshwater aquarium have representatives who have adapted to brackish water conditions.

The cichlid family, for instance, is represented by at least two species of the same genus, Etroplus suratensis, the Green Chromide, and the smaller and more commonly seen Etroplus maculatus or Orange Chromide.
The main representatives of the catfishes are the Arius species or Shark Catfish. This is a family of relatively large species, some growing up to 500 millimetres or half a metre in length which, in ‘old money’, is getting on for twenty inches.

A myriad of different Goby species is available for the brackish water tank. Most commonly seen of these are probably the Knight Goby, Stigmatogobius sadanundio, and the Bumblebee Gobies, Brachygobius doriae, Brachygobius nanus and Brachygobius xanthozona. All these Gobies are very attractively coloured and are a delight to view, scurrying around the aquarium with their jerky movements.
Knight Goby,
Stigmatogobius sadanundio

The Bumblebee Gobies, as their common name would suggest, are black and yellow striped, while the Knight Goby has a base colour of grey covered with very bright and attractive mid-blue spots.

Some species of Puffer Fish and some Rainbowfish are well-suited to the brackish water aquarium.

There is the Scat, sometimes known as the Argus Fish, Scatophagus argus, the Monos, Monodactylus argenteus, and the larger, deeper bodied and less commonly seen Monodactylus sebae (pictured).

The movement of Monos around the tank seems to indicate a constant struggle for supremacy between nervousness and grace. There are the Archer Fish, Toxotes species, so named for their ability to spit a powerful jet of water at insects paused in a false sense of security on plants overhanging the water surface. The startled insect is thus knocked into the water and becomes a meal for the hidden marksman. What a marvel it is that this fish is able to calculate and allow for the refraction of the light through the water in a split second and almost unerringly find its target.

The number of different species that you can keep in brackish conditions is considerable; there are far too many to mention in detail here.
Generally, the shopkeeper can probably advise you on which species are brackish which are not and, as with fresh water and marine fish, knowing the water conditions to which the fish are acclimatised in his or her tank is important.

Another good source of information is, of course, reference books.
These will tell you the preferred conditions of the particular species featured. One word of warning, I have seen mentioned in some publications the words ‘will tolerate’ salt or brackish conditions. I assume the term ‘will tolerate’ to mean that these are not necessarily the fish’s favoured conditions but the fish can be forced to adapt to them. This can neither be right nor fair to the fish and is totally unnecessary since there are many true brackish water species readily available to choose from.

The keeping of various brackish water fishes is sometimes ignored or pushed to one side as ‘something to have a go at one day’. Yet in reality, it can be a fascinating branch of our hobby and well worth consideration for the ‘here and now’. The care and maintenance of brackish water fish are no more difficult or complicated than the tropical freshwater fishes that most aquarists keep, and the wide diversity of unusual and interesting species available makes it well worthwhile and rewarding.

Last updated September, 2006