Where Do Brackish Water Habitats Exist?

The environments from which brackish water fishes are collected are diverse. Estuaries are the best known. An estuary is the part of a river where it meets the sea. Typically, estuarine waters are slow and sluggish, and often very silty and fertile. As a result they are not always as attractive to look at as the clear waters of a mountain stream, but they are tremendously productive. One characteristic of estuarine ecosystems, and brackish water habitats in general, is that while productivity (the amount of food available) is high, diversity (the number of species) can be quite low compared with rivers or the sea. This apparent contradiction is because relatively few fish and invertebrates can tolerate the fluctuations in salinity. On the other hand, those animals that can live there do so in enormous numbers. Gobies, flatfishes and catfish are characteristic fishes of estuaries, ranging from the fresher waters upstream right down to the sea. A few freshwater fishes may occur in the least saline parts of the estuary (for example garpike, ropefishes and cichlids). Many marine fishes, especially as juveniles, inhabit the saltier part of the estuary (such as sea bass, flatfishes, tarpon and herring).

Another important brackish water habitat is the mangal (or mangrove swamp). Mangals are characteristic of the tropics. Some mangals develop in estuaries, while others fringe islands. Mangrove plants are some of the most remarkable plants to be found anywhere in the world. They form dense forests which support a tremendous variety of animals both above and below the surface of the water. In the canopy, monkeys, snakes and a huge variety of birds are to be found; while the aquatic roots support oysters and barnacles, as well as many fish, snails and crustaceans which live in and around the thick knots of roots and stems. Mangals have provided many good fish for the aquarist, such as mudskippers and archer fishes. The mud that collects within the mangrove roots is a prime habitat for fiddler crabs, various clams, and snails.

The temperate equivalent of the mangal is often said to be the salt marsh. These are tidal habitats, periodically covered by the sea. Salt marshes are also important habitats, especially as the feeding grounds for seabirds and fishes. Instead of trees, the major plants are grass-like. Gobies and sticklebacks are especially common in salt marshes, which are quite easy to explore and a good place to collect native fish and invertebrates for the aquarium. Unfortunately many salt marshes are threatened by agricultural or commercial development.

Common to both the tropical and temperate zones, seagrass meadows are found in shallow water. Seagrasses are true plants and not algae, but have proved to be difficult to keep in aquaria. There are many fishes and invertebrates found in sea grass meadows, perhaps the best known is the seahorse. The salinity of a seagrass meadow is rarely much below that of full strength seawater, but seagrasses do occur in lagoons and estuaries where the water is brackish.

What is Brackish Water?

Brackish water is water which contains more sea salts than freshwater but less than the open sea. Moreover, brackish water environments are also fluctuating environments. The salinity is variable depending on the tide, the amount of freshwater entering from rivers or as rain, and the rate of evaporation. As a result many brackish water fishes are tolerant of changes in salinity, and in fact many positively benefit from similar periodic changes in aquaria.

On the other hand, filter bacteria will not tolerate big changes in salinity. So in the aquarium it is normal just to make changes a point or two up or down the specific gravity scale, i.e., from SG 1.005 to 1.006, or down from 1.012 to 1.010.

Water Temperature & Chemistry

Most tropical brackish water fish will do at temperatures of 20 to 25° C (68 to 77° F). A few species appreciate slightly warmer conditions, for example archerfish and bumblebee gobies prefer the temperature to be not less than 25° C (77° F). When using an aquarium heater, ensure that you are using one that is safe in salt water -- some are not.

Fishes from the temperate zones will not do well at tropical temperatures for long. While many species will adapt to ambient room temperature provided the oxygenation of the water is adequate, other species may require a chiller, particularly in summer. Aim for a temperature of around 15° C (59° F) and certainly no higher than 18° C (65° F).

Fish from slightly brackish water (such as kribensis and spiny eels) are generally not picky about pH or hardness provided extremes are avoided. A slightly acidic to neutral pH with low to moderate hardness appears to suit these fish best. Species from the more marine end of the spectrum, such as monos and scats, need hard, alkaline water conditions. Sea salt contains minerals that should harden the water and make it more alkaline, but at low salinities sea salt alone may not be adequate. A good way to ensure a high pH and hardness is to use calcaereous media in the filter, such as coral sand or crushed coral.

Using these as a substrate for an undergravel filter does work, but over time bacteria and algae coat the grains reducing their effectiveness. It is more reliable to place the calcareous media in a canister filter where it can be more easily removed, cleaned, or replaced.

Sea shells will also help to buffer the water. Do not use snail shells from the garden, garden pond, or anywhere else that you cannot be sure the shells have not been exposed to poisons such as insecticides and herbicides. Shells from the fishmongers or grocery store (such as oysters, scallops, and clams) can be used but clean thoroughly beforehand. Small quantities of meat are often left in the shell even after the shellfish has been prepared for eating, and these will decay in the aquarium. Small fish such as gobies are especially fond of shells and will use them as resting places and spawning sites.

Do not use dead corals in brackish water aquaria. Besides not being a natural part of brackish water ecosystems, many corals are threatened by over-collecting (among other pressures) and there are perfectly adquate artifical alternatives.

Bogwood can be used in brackish water aquaria, but it does tend to acidify the water. In tanks containing hard water this effect will be minimal, but the tannins released by the wood will still tint the water yellow or brown. This is not intrinsically bad, and some fish, such as glassfish, look especially nice in slightly coloured water.

Salinity & Specific Gravity

I have written a computer program called Brack Calc that convert between salinity, specific gravity, and salt concentration. You can download Macintosh and Windows versions of this program here.

Sea salt is more than just sodium chloride, it also contains calcium carbonate and bicarbonate, various metal salts, and small amounts of phosphates, sulphates and nitrates. For this reason adding table (cooking) salt is not an adequate substitute. Neither are the aquarium tonic salts sold for community tanks. The best way to make brackish water is to use proper seasalt (marine mix) sold for reef aquaria.

If you have a marine aquarium, good quality water from the marine aquarium can be used in the brackish water aquarium. To make half-strength seawater for example, you would mix old marine aquarium water with fresh tap water in equal amounts. Always remember to add dechlorinator. The ‘old’ water from the marine aquarium needs to be of reasonably high quality, so check the nitrate level to make sure that it is not excessively high. Nitrate levels up to 50 mg/l will be tolerated by most brackish water fish without complaint.

Salinity is a measurement of how much salt is dissolved in the water. It is difficult to measure, so most aquarists prefer to describe the saltiness of the water as a density. The density of a substance is how heavy (how much mass it has) per unit of volume. Pure water has a density of 1.000, i.e. one litre of water weighs one kilogramme. Seawater is denser as it has salt dissolved in it. Typically seawater has a density of around 1.025, which means one litre of seawater weighs 1.025 kilo grammes.

Hydrometers and refractometers

To measure the density of the water you will need a hydrometer, a device that measures the density of a liquid. Most hydrometers sold for aquarists are calibrated to be accurate at around 20 to 25° C. Swing arm hydrometers are commonest. These are small plastic devices, which are filled with the water to be measured. In doing so an arm moves inside the device which points at a scale, from which the density can be read. Cheaper, but less often seen, are floating hydrometers. These look a bit like glass thermometers with a weight at the base and a scale running up the glass. These are also sold with brewer’s kits for people who make wine and beer. So long as the scale runs from 1.000 to 1.030 or so then they are fine. Floating hydrometers are placed in the tank or bucket of water to be tested and left for a moment. When the device settles, the surface of the water will cover the scale up to the density which can be read off.

The density of water required varies depending on the fishes kept. Fishes from the fresher end of the range (like rainbowfish and kribensis) enjoy a density of less than 1.005, as do most freshwater plants. Fishes which live mostly in from the sea, like dog face puffers and batfishes, prefer a salinity of over 1.010. But most brackish water fishes can tolerate the entire range: from 1.000 to 1.025. These include such fish as sailfin and black mollies, sleeper gobies, monos, scats, archerfish, and shark catfishes.

If you can’t get a floating hydrometer which shows the entire range the alternative is to use a swing arm hydrometer and dilute full strength seawater. For example, if you want one-tenth seawater, or about 1.002 to 1.003, you would make full strength (1.025) seawater and then dilute it with freshwater in the ratio one part salt to nine parts fresh. One half seawater, about 1.012, would be made by mixing one part salt water to one part fresh.

What about the hand-held refractometers now widely sold online for around $50? On paper at least, these appear to be more accurate than hydrometers. In practise though, consumer-grade refractometers are not comparable with lab instruments. A hand-held scientific-grade refractometer would cost aroudn $1000, and desktop instruments even more. Are the cheaper, consumer-grade refractometers even useful? Yes, they work at least as well as good quality hydrometers. Both refractometers and hydrometers will only be accurate if maintained and used correctly. But it is far from clear that refractometers are actually better than hydrometers, as this discussion over a Reef Central illustrates.

Can you measure out the salt in teaspoons or tablespoons?

Measurements like one teaspoon per litre are sometimes mentioned. One teaspoon of salt weighs about 6 grammes, so very roughly, you can estimate the amount of salt using teaspoons. Water at SG 1.005 for example contains 9 grammes of salt per litre, so for a 10 litre bucket, you’d add 90 grammes of salt, or 90/6 = 15 teaspoons.

Making up your water

However you measure your salt, it is important never to add the salt directly to the aquarium. Not all the salts dissolve at the same rate, and some can damage your fish when undissolved. Always make the salt water up separately, following the instructions on the package carefully. Allow the salts to fully dissolve, there should be no residue at the bottom of the container. Stirring helps, but best of all use an airstone to bubble the water for twenty minutes or so.

Salt water is highly corrosive. Most metals corrode quickly even when splashed by salt water. It is therefore important to make sure that no metal objects are used inside the aquarium, and metal hoods and stands should be avoided.