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Brackish Aquariums

By Neale Monks

Brackish water habitats occur where freshwater meet the sea, and it is therefore not surprising that keeping brackish water fishes combines elements of both the freshwater and marine branches of the hobby. On the one hand, a brackish water tank is set up in much the same way as a freshwater tank, and in fact a freshwater tank can be converted to a brackish water one very easily indeed. But on the other hand, brackish water needs to be made up using marine salt mix, and measuring the saltiness of the water involves the use of a hydrometer or refractometer, devices more normally associated with marine fishkeeping. In addition protein skimmers can work almost as well in brackish water as they do in seawater, giving the brackish water aquarist an extra tool for water quality control than freshwater aquarists are denied. It's because of this mix of freshwater and saltwater aquarium characteristics that brackish water aquaria have often been considered a good stepping-stone between the two hobbies. Certainly, an aquarium set up for high-salinity brackish water fish such as Shark Catfish, Monos, and Scats can easily be converted to a fish-only marine aquarium, the brackish water fishes being perfectly happy being maintained at normal marine salinity.

What Salinity?

Some habitats, like coral reefs, South American blackwater streams, or Lake Malawi rocky shores, can be easily tied down in terms of temperature, pH, hardness, and salinity. Doing this is with brackish water is much more difficult, because by definition they are changeable habitats occurring in a variety of places such as coastal streams, lagoons, estuaries, and mangrove swamps. In short, there is no one thing called "brackish water". However, not all brackish water fishes necessarily tolerate the full range of salinities from pure freshwater through to fully marine conditions. Broadly speaking, brackish water fishes can be divided up into three main groups: low salinity species, medium salinity species, and high salinity species.

While primarily a freshwater fish, bumblebee gobies tend to thrive in slightly brackish water.

Low salinity brackish water can be defined as water with a specific gravity (or SG) between 1.002 to 1.005. Two sorts of fish can be kept in low salinity water: true brackish water species that need slightly brackish water, and species that are really freshwater fish but happen to be tolerant of slightly brackish water. Among the true, low salinity brackish water fish are Orange and Green Chromides, Figure-8 Pufferfish, Pike Livebearers, and Knight Gobies. Freshwater fish that happen to tolerate brackish water include Spiny eels, Kribensis, Glassfish, the archerfish Toxotes microlepis, and Florida Flagfish. Bumblebee Gobies and Mollies are difficult to place in either group, because in the wild both occur primarily in fresh water. However, in aquaria, they seem to do better when kept in slightly brackish water.

Mid-salinity brackish water can be defined as species that can be kept successfully at anything from about SG 1.008 right up to full strength seawater at SG 1.025, but with a middling value of SG 1.010 being more than adequate for long term health. Classic examples of mid salinity brackish water fishes are scats, monos, certain archerfish such as Toxotes jaculatrix, Colombian shark catfish, and violet gobies.

High-salinity brackish water is typical of mangroves and estuaries where the influence of the sea is strong. The fishes that live here may well tolerate low salinity brackish water, even freshwater, for a while, but in an aquarium such fish should not be maintained at less than about SG 1.012. Many of the fishes found here are only transient residents in brackish water, and come in from the sea, either to hunt for food or to breed. A surprisingly wide variety of marine fish, even ones associated with coral reefs or the open seas, spawn in brackish water and as juveniles mature in estuaries and mangroves. Among the species sold as aquarium fish, dog-faced puffers, milk-spotted puffers, and many of the snappers (including Lutjanus argentimaculatus, Lutjanus apodus, and Lutjanus sebae) fall into the category of marine fish with a high tolerance of brackish water, particularly when young.

There is another good reason to choose a salinity and stick with it, and that's because the biological filter bacteria cannot tolerate big changes in salinity. Though the science is a bit vague, as a rule of thumb a filter matured in a freshwater aquarium can be adapted to a salinity about 25% that of normal seawater (SG 1.005). Raising the salinity above that stresses freshwater-adapted filter bacteria, and there's a process of re-maturation that has to occur as the freshwater bacteria die off and the brackish water bacteria start multiplying. It isn't known for certain whether the brackish water filter bacteria are the same ones as in marine aquaria, though taking saltwater-adapted filter bacteria and dumping them in brackish water below SG 1.015 and expecting them to survive doesn't seem to work. Bacteria adapted to a middling salinity, around SG 1.010, seem to be somewhat tolerant of changes around this level, and you can vary the salinity within the range SG 1.008-1.012 without problems. There is some anecdotal evidence that varying salinity encourages some brackish water fish to spawn, but beyond that, doing so isn't necessary.

Making Brackish Water

Brackish water is made in the same way as salt water is made for a marine aquarium, except less salt is used. Marine salt mix must be used, not aquarium tonic salt or cooking salt. The brand of marine salt doesn't matter in the least, so use Instant Ocean, Reef Crystals, Kent Sea Salt, or whatever brand happens to be cheapest in your neighborhood. Brackish water fishes have, by definition, an ability to adapt to variations in water composition, so switching from one brand to another doesn't matter either. Generally speaking, you don't need to worry about pH and hardness, either, as the addition of marine salt to the water should take care of these two parameters. That said, you're aiming for a pH between 7.5-8.0 in most cases, and a hardness level that is "hard" to "very hard" on whatever scale you're using. 

In approximate terms, tropical aquarium seawater at SG 1.025 contains about 4.7 ounces of marine salt per US gallon (or about 35 grams per liter). So medium strength brackish water for a tropical aquarium at SG 1.010 will have about two-fifths that quantity of salt per gallon, or about 2 ounces of marine salt per US gallon. Low salinity brackish water at SG 1.010 will have even less salt, about 1 ounce of marine salt per US gallon. However, while measuring salt by weight is fine for estimating how much salt to buy and use, you will still need to measure the specific gravity of the water once you've mixed in the salt. This is because an open box of marine salt mix absorbs moisture from the atmosphere, with the result that after a while, a significant amount of each quantity of salt you think you're adding to each bucket of water is in fact water as well! It should go without saying that methods of making up brackish water by adding a certain number of teaspoons or tablespoons of salt per gallon of water are even less reliable. 

Always make brackish water in  a bucket and only then add it to the aquarium. Never add salt directly to the aquarium. Use water close to the temperature of the aquarium. Stir the salt thoroughly, and give the water 10-20 minutes to fully dissolve all the salt. Stir the water again, and check for undissolved particles. Finally, use a hydrometer or refractometer to check that the specific gravity of the brackish water is correct. If it is, then the water is ready to use.

 Hydrometer or Refractometer? 

While marine aquarists enjoy debating the relative merits of these two devices, for the brackish water aquarist the answer is very simple: use which ever one you like. A low cost floating glass hydrometer (costing around $5) is just as serviceable for the brackish water aquarium as a fancy refractometer (which will cost $40 or more). Brackish water fishes, by definition, don't need precise salinities, and all your filter bacteria want is a steady salinity, the actual value doesn't matter much. In other words, the error of margin resulting from the use of a even a cheap hydrometer will be well within the tolerances of the livestock being kept. This isn't true for reef tanks where sensitive fish and invertebrates are being kept, hence the arguments of which specific gravity measuring device is best.

Four-eyed fishes of the genus Anableps don't actually have four eyes, but rather specialized divided corneas that allow them to see two images at once.  These fishes can use this specialized vision to see what is going on both above and below the water by hovering right at the surface.

 One thing should be mentioned though: all these devices only work reliably when used properly. If you're going to use a floating glass hydrometer, always check it's one calibrated for use at 77˚F (or 25˚C). Some hydrometers are designed for use at different temperatures, and are consequently unreliable when used at the temperature of a tropical aquarium. Floating glass hydrometers can be tricky to read, but there are really only two things to remember about them. Firstly, they only work when the water is still. So instead of sticking them in the tank, pour some aquarium water into a large glass jar, let it settled for a moment, and then put the hydrometer in there. Secondly, be sure and ignore the meniscus (the bit of water that seems to creep up the stem of the hydrometer) -- the specific gravity should be read off at the level of the water, not the meniscus. It is also a good idea to take care to clean and store the hydrometer properly. So when you're done, rinse it off in warm water to prevent any build up of lime or salt, and then store it somewhere safe so that it can't be damaged.

Decorations and Plants

A low-salinity brackish water aquarium can be treated as a freshwater aquarium for most practical purposes. Gravel, silica sand, slate, bogwood, and ceramic ornaments will all work fine in such a tank. Practically all aquarium plants tolerant of hard, alkaline water will also do well in slight brackish water at SG 1.003. A small but serviceable selection will thrive at SG 1.005 since they naturally inhabit brackish waters in the wild, including Java fern, Egeria densa, Bacopa monnieri, Cryptocoryne ciliata, Samolus valerandi, and Crinum calamistratum. Moss balls and Java moss also do well at SG 1.005 as well. Of course, these plants will only thrive if their other needs are met as well, and species like Bacopa monnieri and Samolus valerandi are quite demanding plants in terms of lighting and substrate quality. On the other hand, Java moss and Java fern are much less demanding, and can both be used very effectively in small planted tanks housing things like gobies.

In higher salinity aquaria plants become problematical, as very few freshwater plants will do well above SG 1.009, the notable exceptions being Java fern, which has been taken up to SG 1.013, and Samolus valerandi, which has been adapted to waters as salty as SG 1.020! These exceptions aside, once you get above SG 1.005 it is simply much easier to either use plastic plants or forget about plants altogether. Plastic algae- and kelp-type plants are especially good in this context, being exactly the sort of thing that you'd see in an estuary or mangrove. Mangroves are another option, but since most of their growth is above the water line, they aren't that useful for the average aquarium. 

For decorating a mid to high salinity aquarium, it's best to concentrate on rocks, shells, and artificial ornaments such as tree roots. Bogwood can be used, but it will stain the water brown, and it can reduce the pH under certain circumstances. In most cases bogwood is safe, and any acidification will be buffered (inhibited) by the calcareous carbonate in the marine salt mix, but if a lot of bogwood is used, and only small water changes are performed, the bogwood might overwhelm the buffering capacity of the brackish water. That said, a mass of bogwood can create a lovely mangrove forest look, and fish like Archerfish and Pufferfish seem to enjoy being able to hide in the shadows when they want to.

Substrate fauna hunters like these Awaous flavus require proper substrates to accommodate their sand-sifting behavior.

In a low salinity aquarium, the best substrate to use is plain gravel, which may be augmented with laterite or aquarium compost if good plant growth is required. Silica sand is a good alternative, but works best with fishes that will turnover the sand and keep it nice and clean, for example gobies and flatfish. A mix of plain gravel and silica sand can create a very authentic reproduction of an estuarine river bed, especially if a few broken snail shells and fragments of oyster shell are mixed in as well. The addition of a few water-worn boulders will further enhance the illusion. In higher salinity aquaria, mixing sand or gravel with crushed coral, broken seashells, and/or plain coral sand, can create an authentic substrate that will additionally raise the pH and increase the buffering capacity of the aquarium. In tanks where the SG is above 1.010, and when fishes that need a high level of hardness and pH are being kept, then a substrate consisting entirely of coral sand and/or crushed coral should be used instead. 

While adding things like dead coral fragments and large seashells will do no harm, it is questionable whether they are authentic additions to the brackish water aquarium. If in doubt, stick to things that naturally occur in estuaries and mangroves, such as oyster and mussel shells, small clam shells, and small, nondescript snail shells. Anything obviously marine, like spiny conch shells and giant clam shells, will shatter the illusion that what is being created is an authentic slice of a brackish water habitat. 

Livestock: Fishes

Can you see the Brachirus sp. sole in the picture on the left?  It is easier in the picture on the right!  These flatfishes make fascinating inhabitants for the brackish aquarium.

The best thing about the brackish water aquarium is the extraordinarily wide variety of fishes available. The majority of brackish water fishes belong to marine fish families. In some cases, these families are popular with marine aquarists as well, for example Pufferfish, Moray Eels, Damselfish, Snappers, Gobies, and Blennies. But in many cases they come from families that, for one reason or another, are rarely kept by marine aquarists. Good examples of these include Mudskippers, Halfbeaks, Spaghetti Eels, Waspfish, marine catfish, and various types of Flounder and Sole. Some families are more or less brackish water specialists, of which the Archerfish, Scats. and Monos are perhaps the best known and most frequently traded examples. Finally, there are families of freshwater fishes that include species adapted to brackish water, including many species of cichlid, livebearer, and killifish.

Dermogenys sp. and other halfbeeks are among the more unusual brackish fishes in the trade.

When choosing fishes, the same basic rules apply as with choosing fishes for a freshwater or marine aquarium but with one key addition: it is essential to select fishes that tolerate the same level of salinity. There is no point mixing, for example, a low salinity brackish water fish like a spiny eel with a fish that needs high salinity water, such as a scat. It is a good idea to decide whether you're going for a low salinity aquarium (at or below SG 1.005) or a high salinity aquarium (SG 1.010 upwards). Once you know that, it is much easier to select suitable livestock.

Livestock: Invertebrates 

Brackish water habitats teem with invertebrates, but surprisingly few are traded within the fishkeeping hobby. Among the most notable are snails and hermit crabs. True brackish water nerite snails include the olive nerite (Vitta usnea), the Virginia nerite (Neritina virginea), and the zebra nerite (Puperita pupa). Though often sold as freshwater snails, they will in fact do much better in brackish water, ideally between SG 1.005 and full strength seawater. There are a variety of true freshwater nerites sold (mostly unidentified) and while they won't tolerate high levels of salinity, they will probably do well in low salinity brackish water (SG 1.005 or less). Nerites are excellent algae eaters, and the zebra nerite in particular is one of the few snails that seems happy to eat blue-green algae. Malayan livebearing snails (Melanoides tuberculata) also do well in brackish water, up to about half-strength seawater (SG 1.010 or less). The less frequently traded West African snail Pachymelania byronensis is another brackish water snail, though in this case the specific gravity should be below SG 1.005. Generally speaking, the popular apple snails do not do well in brackish water, and nor do things like pond snails.

There are numerous brackish water crabs, but almost all of them are amphibious, and as such cannot be kept in an aquarium. This includes things like the red-claw crabs (Sesarma bidens) and fiddler crabs (Uca spp.) quite widely sold as novelty pets. Although they prefer brackish water over freshwater as a place for bathing and foraging, both need a land area to explore as well. The best truly aquatic crab for the brackish water aquarium is the Blue Leg Hermit Crab, Clibanarius tricolor. Normally sold as a marine invertebrate, it is actually a very hardy and tolerant animal that is known to do well down to about SG 1.010. Its size and sturdiness makes it an excellent scavenger and algae eater for an aquarium containing robust brackish water fishes such as Scats and Monos.


Algae can be a problem in a brackish water tank. Essentially the issue is that brackish water fishes tend to eat a lot, meaning that the water becomes quickly polluted with nitrates and phosphates, encouraging the growth of algae. This is exacerbated by the fact that unless the salinity is fairly low, it isn't possible to have fast growing plants to use up the nutrients and suppress the growth of the algae. So how do you deal with algae in the brackish water tank?

Firstly, you can add plants where possible. At SG 1.005 or less, things like Vallisneria and Hygrophila will do well, and these grow quickly enough to suppress the growth of algae in most situations. Although often touted as algae-busters, mangrove seedlings grow too slowly to make much difference.

Secondly, you can use Nerite snails. Since these can be bought easily and inexpensively, and then adapted to whatever salinity is required, they are an excellent addition to the brackish water tank. The only downside is that they will be eaten by specialist snail-eaters, such as Pufferfish. Blue Leg Hermit Crabs can be used alongside the snails in a high salinity brackish water aquarium.

Thirdly, you can use algae-eating fishes. Many brackish water fishes will eat algae, including mollies, Florida Flagfish, Violet Gobies, and Scats (of course, scats will eat anything!). In a low salinity system (SG below 1.005) there are even a few brackish water plecs, such as Hypostomus watwata and Hypostomus ventromaculatus, but these are very, very rarely traded. Hypostomus plecostomus is known to tolerate brackish water, being firmly established in the brackish water creeks along the coastline of Florida, but this species is actually rather infrequently traded, despite being commonly described in aquarium books. The "common plecs" sold in aquarium shops are usually species of Pterygoblichthys and Liposarcus, and these are not known to be salt-tolerant.

Finally, control the environment so that the algae cannot grown too rampantly. Removing uneaten food, performing regular water changes, and limiting the amount of light will all help to reduce algal growth. To some extent, algae is a natural part of the brackish water environment, and many aquarists simply accept it as such, and removing nothing more than the algae on the front panes of glass.

 Maintenance and Medications

In most ways, maintaining a brackish water aquarium is the same as maintaining a freshwater one, with the added step of measuring the specific gravity periodically and topping up any loss from evaporation with freshwater (not brackish water). Monitoring the pH and hardness is important, perhaps even more than in a freshwater tank because brackish water fishes on the whole have a low tolerance of acidic water. Regular (i.e., weekly) water changes of 20% or more should prevent any problems with acidification, but if your fishes look unhappy, then checking the pH and hardness is a wise first step.

At or above SG 1.010, a protein skimmer will work, and installing one of these will help remove organic materials before they decay into nitrates and phosphates, improving water quality. This is important in tanks where fishes intolerant of high levels of nitrates are being kept, for example Pufferfish.

Few medications or test kits are designed expressly for brackish water. Only use products that are safe for use in both salt and freshwater aquaria, as these will work fine in brackish water aquaria as well. However, because brackish water is generally hostile to both freshwater and saltwater parasites, things like whitespot and velvet are actually very rare in brackish water aquaria. Furthermore, because brackish water fishes tolerate big changes in salinity without problems, performing saltwater or freshwater dips to "zap" external parasites is generally safe and effective.

 Final Comments

Brackish water fishkeeping is something of a secret within the hobby. Rarely mentioned by the standard aquarium books, brackish water fishes usually turn up in tropical fish shops unmarked, with the retailer knowing little or nothing about them. But for the discerning aquarist, the brackish water aquarium provides the opportunity to keep many of the most attractive and interesting species in the hobby. Some are small, like the blennies and gobies, ideally suited to the miniature aquarium. Others are big and impressive, like the scats and Monos. Some exude power and grace, like the Colombian Shark Catfish, while others are just plain comical, like the Violet Goby. Some are personable characters, like the Green Spotted Puffer, others are just plain weird, like the Paradise threadfin. There are fishes that exhibit extreme behaviours, like the four-eyed fish, Archerfish, and Mudskippers, and fishes that with extreme bodies, like the flounders and Spaghetti eels. In every way, brackish water fishes beautifully illustrate the sheer diversity of fishes.

Further Reading

There are surprisingly few books exclusively about brackish water fishkeeping. The following books are the only English-language titles dedicated to the subject (with the Aqualog book also available in German).

Brackish Aquaria, by Michael Gos, published by TFH, 1979, ISBN 0-876665199

Brackish Water Fishes, by Frank Schäfer, published by Aqualog Verlag, 2005, ISBN 3-93602782-X

Brackish Water Fishes, edited by Neale Monks, published by TFH, 2006, ISBN 0-793805643

Brackish Systems on WWM


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