Creating a brackish aquarium

When it comes to setting up an aquarium, the obvious choice is between freshwater or marine. But in fact, there is a third option - a brackish aquarium. A range of fish have adapted to live in this type of habitat, on the borders between sea water and freshwater, close to the mouths of rivers and in mangrove swamps.

Some suitable choices

An unusual fish which can thrive in brackish water is the glassfish (Parambassis ranga). It has a transparent body, enabling you to see its skeleton very clearly. Unfortunately they are not brightly coloured and this has resulted in an unpleasant and cruel practice of injecting them with dyes which show up through their bodies. Although most reputable stores refuse to stock dyed glassfish, they are still occasionally seen.

Another interesting choice can be the archer fish ( species), so-called because of its ability to shoot down insects into the water, spurting droplets at them from several inches away. In the aquarium, archer fish can be persuaded to display this skill with a high degree of accuracy, aiming at food located on the sides of the tank above the water level.

Some brackish water fish have a much more variable appearance in terms of their coloration. Scats (Scatophagus argus) may range in colour from silver through shades of red to gold, while their markings can vary from spots to stripes. They thrive if kept in small shoals of about five fish. Scats will grow to about 15cm (6in) long in aquarium surroundings.

The mono (Monodactylus argenteus) has a flattened body shape and is silvery,Mono with yellowish fins. Black stripes run through the eyes and also across its gill covers. Various species of boxfish are another possibility to consider for a brackish aquarium, although they can be quite aggressive and are best kept on their own. They have a rather square, box-like shape as their name suggests.

More peaceful by nature is the bumble bee goby (Brachygobius xanthozona), which only grows to about 3.75cm (1.5in) long. It can be shy though and will appreciate somewhere to hide away on occasions. It is so-called because of its black and yellow markings. None of these fish is difficult to look after and they will thrive on the range of foods stocked by aquarist stores. As with all aquarium fish, it is important to feed them two or three times daily rather than just providing one large meal.


The corrosive effects of salt means that it is important to use an all-glass aquarium rather than one of the old style metal frame tanks. Similarly, choose a plastic rather than a metal hood for the aquarium. Covering the tank is important, not just to provide the aquarium with lighting but also to prevent evaporation of water which can occur at a fast rate in centrally heated homes. This in turn will cause the salt content of the remaining volume of water to increase.

It is quite easy to make up the water for a brackish aquarium. You can use one of the special blends of marine salts now available. But it is always advisable to treat the water with a dechlorinator prior to adding the marine salt. This is because these chemicals will often not work as effectively in salt water, which could have fatal consequences for the fish.

Instructions for mixing the salt will be given on the packaging. You will need to stir a specific quantity of the salt in with ordinary tap water, ensuring that it has dissolved completely. Carrying this out in a bucket before adding water to the aquarium is to be recommended.

Measurement of the salt content of the water is carried out using an instrument called a hydrometer, which will give you a specific gravity reading. You can buy a hydrometer from aquarist outlets, along with the sea salts. In the case of a brackish aquarium, a specific gravity figure between 1.005 and 1.015 is required.


It will probably be better to aim for the lower end of this scale if you want to include plants in the aquarium, as most aquatic plants will not grow in salty water. There are a few which can prove sufficiently adaptable for a brackish environment. These include Java fern, which will root on submerged objects including rocks. Tie this plant in place first so as to anchor it down in position until it has established itself on the rockwork. You can then cut off the support later.

Java fern will also grow well on bogwood, which is available from aquarist stores. But it is vital if you use this wood, either in a brackish or freshwater aquarium, to treat it first. Otherwise it will turn the water a dirty shade of yellowish-brown. Soaking the bogwood in a bucket of water and changing the water regularly will help to leach out this unwanted colour over perhaps a week or so.

The other alternative is to paint the bogwood carefully with polyurethane varnish. This will seal the surface but you will need to wait until it has dried thoroughly before submerging the wood in the aquarium. Several coats of varnish may be required to achieve a complete seal.

You may be able to obtain pieces of mangrove roots as well, which can be very effective as decor. It is also possible to obtain mangrove plants which can be incorporated into the tank too, certainly while they are small. These are often advertised on eBay.

Vallisnerias are a more traditional group of aquarium plants prove sufficiently robust to survive in brackish surroundings but they will rarely grow as well here as they would in freshwater. You can of course simply add plastic substitutes as an alternative but including at least some live plants will help to utilise the nitrate produced from the fishes' waste in the aquarium.

The floor covering can be made up using a mixture of gravel and sand. The large pieces of gravel will help to keep the sand from clogging up together, which is especially important if you have an undergravel filter. Otherwise the filter bed will become blocked and the filter will fail to work effectively.