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The Brackish Waters of the Bayou:

Native brackish water fish for the home aquarium


by Neale Monks


The North Atlantic coastline running south from North Carolina through to the Gulf of Mexico contains a rich variety of estuaries, mangroves, salt marshes, and seagrass meadows that provide a huge variety of fish suitable for the home aquarium. Many have become hobby staples, such as livebearers black and sailfin mollies, dragon and sleeper gobies, and the ever-popular Florida flagfish. If you happen to live in this part of the world, collecting these fish is a fun way to expand your hobby into a wider appreciation of our native wildlife, but naturally you do need to make sure that it is legal to collect these species in your area. There are often restrictions on collecting live fish in state and national parks, for example. 

Even if you cannot catch your own, these native species are so widely traded, both in the United States and around the world, that setting up an American-themed brackish water aquarium isn't difficult at all. In fact, most of these fish are remarkably hardy (even by the standards of brackish water fish!) and many species are small as well, making them ideal inmates for a second aquarium to complement an existing community tank you might already have. In this article, we'll take a look at these North American species and think about the best way to create the right conditions for them in the home. Though aquarists are often moved to create aquaria based on the African Rift Valley lakes or the Rio Negro in Brazil, there's really just as much fun to be had looking at something a bit closer to home!



The livebearers are perhaps the most characteristic of all the North American brackish water fish, and so make a logical place to start this essay. Most are small, typically less than 10 cm (4 inches) in length, and almost all feed on a mixture of insect larvae, small aquatic animals like water fleas, and algae, making them very easy to adapt to aquarium life. 

Far and away the best known of all the North American livebearers is probably the common mosquitofish, Gambusia affinis. These fish are most abundant in the Deep South from Florida across to Texas and down into Mexico. Mosquitofish prefer still, shallow waters thick with vegetation and tree roots, where they are relatively safe from larger, predatory fish and have constant access to a rich supply of the insect larvae and other small animals they life to feed on. They can be found in both fresh and brackish waters, and will adapt well to an aquarium with a specific gravity of up to 1.010. Other water parameters do not seem to be important, and in the wild at least these fish are commonly found in stagnant water, and the exact pH and hardness doesn't matter provided extremes are avoided. Mosquitofish resemble guppies in size and shape, and as with guppies, the male is much smaller than the female; the most notable difference is the absence of the brightly coloured tail fin. There are a variety of subspecies, and while the females are all pretty similar, the males vary in the size and number of the black spots and blotches they have covering the body. In the most commonly traded subspecies, Gambusia affinis holbrooki, often called Holbrook's mosquitofish, these patches are large and often merge, giving the male fish a distinctly piebald appearance. All the mosquitofish are a little on the aggressive side compared with guppies, and some specimens become fin-nippers as well. Consequently, they are best combined with larger fish that are able to look after themselves but are not sufficiently predatory to view the mosquitofish as food. 

Often confused with the common mosquitofish, the dwarf mosquitofish Heterandria formosa is a peaceful little fish that is best kept in a single species aquarium. This is a small species, with even the females barely exceeding 1.75 inches (4.5 cm) in length, making it eminently suitable for a small aquarium containing just a few gallons of water. Occurring throughout the Carolinas and Florida, this is another species that is most commonly found in shallow, weedy waters but unlike the common mosquitofish it doesn't usually inhabit brackish water habitats. While it will adapt to slightly brackish conditions (with a specific gravity of less than 1.005) there is no particular need to add salt and it may well do better without. 

In contrast, the sailfin molly, Poecilia latipinna, definitely does do best when kept in slightly brackish water. Native to the Gulf of Mexico region from eastern Texas up to North Carolina, it has since become established throughout the warmer parts of the United States thanks to accidental and deliberate releases in California, western Texas, and elsewhere. Compared with the common and dwarf mosquitofish, the sailfin molly is a giant: females can reach lengths of up to 4.5 inches (12 cm) in the wild. Oddly, these fish rarely get so big in home aquaria, possibly because they are kept in cramped conditions that do not suit them perfectly well. Again unlike the mosquitofish, the sailfin molly is essentially omnivorous, and prefers a mixed diet containing algae as well as insect larvae and other small animals. In captivity this is easily dealt with by using vegetarian flake foods especially designed for use with mollies and other fish, such as cichlids, that enjoy their greens. Mollies will adapt to a variety of salinities, but an aquarium kept at a specific gravity of around 1.005 is just about ideal; but as with most brackish water, occasionally varying the salinity up or down does no harm at all, and indeed my help suppress external parasites and stimulate breeding.



There are lots of killifish known from the South Eastern United States, but relatively few are regularly traded as pets. Strangely enough, species of the diverse genera Cyprinodon and Fundulus are rather more commonly sold to schools and universities as laboratory animals, and you may well find it easier to obtain these fish by visiting the web sites of biological supply companies than through your local tropical fish store. The sheepshead minnow, Cyprinodon variegatus, is one of the species widely traded this way. It is rarely found in freshwater, preferring brackish and marine conditions, and is also tolerant of hypersaline waters and pools containing unusual mixtures of salts and minerals which other fish cannot deal with. This physiological adaptability is one of the reasons why so many ecologists and biologists like to study this animal. Cyprinodon and Fundulus species generally need quite a large, uncluttered aquarium because they tend to be active and often fairly aggressive towards one another, despite the fact that they typically form loose schools and need to be kept in groups. 

Pretty well the only North American killifish that is truly easy to find in tropical fish stores is the Florida flagfish Jordanella floridae, so named because it resembles Old Glory and comes from Florida, and not because it looks like the flag of the State of Florida. The Florida flagfish is a colourful little fish that only grows to about 2.5 inches (6 cm) in length. Like the sailfin molly, it needs a diet that includes both plant and animal foods, and one secret of success with this species is to allow green algae to grow on the rocks and sides of the aquarium so it can graze when it wants to. Though territorial, the males do not hold huge territories, and provided they are not crowded and each has a cave or shell to call home, several can be kept in a tank without problems. Both sexes have the same basic colouration, but the males generally have the brighter hues, especially at breeding time. While the Florida flagfish will adapt to hard, alkaline freshwater, it does much better in slightly brackish water, a specific gravity of 1.002 to 1.005 being ideal. At these low salinities, plants can and should be used; in particular Vallisneria and Sagittaria are usually quite tolerant of salt provided there are in a rich soil and offered plenty of light. Florida flagfish can be quite shy, and the use of plants helps to settle them in quickly and encourages them to swim about in the front of the aquarium where they can be better observed. 

A North American killifish that is traded far less frequently but is still worth looking for is the diamond killifish Adinia multifasciata (often referred to as Adinia xenica, an obsolete name). This handsome fish has a distinctly rhomboid shape (hence its common name), and the males are patterned with narrow vertical brown and cream stripes all along the flanks. Like the Florida flagfish, this is an omnivorous fish that needs both small live and frozen foods like Artemia and bloodworms as well as algae, blanched lettuce, and other green foods. It isn't picky about water conditions though at least slightly brackish waters are preferred; a specific gravity anywhere from 1.003 to 1.010 can be used, depending on the plants and other fish in the aquarium.



Over the years gobies have steadily become more widely kept and bred, probably because they tend to be quite, accommodating fish that adapt well to small aquaria. Two North American gobies are regularly seen in pet shops around the world, the spotted sleeper goby, Dormitator maculatus, and the violet (or dragon) goby, Gobioides broussoneti. Both are found in fresh, brackish, and salt waters all along the eastern seaboard from the Carolinas through Texas and along into Mexico. 

The spotted sleeper goby is a big, predatory goby that only really works well with other big but peaceful species. It gets along very well with fish like garpikes, the quieter cichlids, and brackish water catfish like Colombian sharks. The problem with this fish is that it is invariably sold at a very small size, often 6 cm (2 inches) or so, when it looks adorably cute and harmless, and gives no clue to the 30 cm (12 inch) monster it can become. At all stages in its life cycle it is remarkably adaptable as far as water conditions go, but as with most of these euryhaline brackish water fish, mature fish generally do best at higher salinities. A specific gravity of around 1.003 to 1.005 is adequate for younger fish, and adults will do well in anything up to full strength seawater. One characteristic of this fish are its eyes, which often appear to be 'glazed', and this is often said to be the reason why these fish are called 'sleeper gobies'. Spotted sleeper gobies are among the more responsive aquarium fish, and if kept properly become very tame, and can even be hand fed. Like all gobies, they appreciate a mix of rocks and plants to hide among and plain sand for digging in, but beyond that, they are not at all demanding pets. Feeding is not usually a problem, as these fish will accept all kinds of live invertebrates such as earthworms and mosquito larvae, as well as small pieces of squid, clams, and prawns. Incidentally, if the spotted sleeper is a little too big for your tastes, the African sleeper goby, Dormitator lebretonis, only gets to about 10 cm (4 inches) in length and will mix well with livebearers and killifish of similar size. It's quite commonly seen in tropical fish shops these days, and looks similar enough to the spotted sleeper goby to provide a fine substitute without disrupting our North American theme too much. 

While the spotted sleeper is a typical, if big, goby as far as looks go, the violet goby is just plain weird. Resembling a cross between a Polypterus and a skinned eel, the violet goby is something of a gentle giant. Despite its fearsome appearance, it isn't particularly predatory, and in fact normally ignores any smaller fish around it. Instead, it uses its large mouth to sift sand and mud in search of the small invertebrates it likes to eat. In fact in busy community tanks this fish rarely gets enough to eat, and anyone looking after violet gobies needs to take care that this bottom feeder gets a fair share at feeding time. Bloodworms, small earthworms, river shrimps, and catfish pellets can all make useful contributions to the diet of violet gobies. They also need some algae, too, and this is most easily dealt with by allowing algae to grow on the rocks or substrate. These peculiar gobies are surprisingly adept at scraping algae from rocks with their teeth. Sinking algae wafers, such as those used for plecs and other suckermouth catfish, are also enjoyed by these fish. Like most gobies, the males are territorial, and it is important that these fish are kept in a tank commensurate to their size, potentially up to 60 cm (24 inches), though in captivity they rarely get even half as big. Apart from ensuring the aquarium has a soft substrate of coral or silica sand, or at least very fine pea gravel, these fish are not terribly demanding with regard to decorations or plants. A few caves will be appreciated though, and most tropical fish stores offer a variety of such structures made primarily for catfish and other nocturnal species. They are not at all fussy about salinity though, and a specific gravity between 1.003 and 1.015 will suit them well.



Among the most bizarre of all the fishes kept by aquarists are the flatfish, and one species, Trinectes maculatus, has becomes reasonably widely traded over the years. Sometimes called the freshwater flounder or the hogchoker sole, this is a medium sized fish that grows to around 15 cm (6 inches) that is most commonly found in shallow brackish and salt waters between South Carolina and Texas. Although predatory, it isn't a threat to anything bigger than a guppy, and in fact does perfectly well on a diet of live and frozen invertebrates including bloodworms, earthworms, and small river shrimps. Like all the flatfish kept in aquaria, this species cannot compete effectively with boisterous fish like cichlids and catfish, and is best kept either alone or with peaceful fish that do not take food from the substrate. It is also a fairly nocturnal species, and during the day spends most of its time hidden under a thin layer of sand with just its eyes showing; but once settled in, it becomes more accommodating, even friendly. They are also able to change their colour and pattern rapidly, to the degree that a well-hidden fish is practically invisible. Flatfish generally have a reputation for being relatively intelligent fish, at least when they are happy and feeding well, and will swim to the front of an aquarium at feeding time. Like the better-known pufferfish and spiny eels, these fish become more rewarding the more effort you make to help them feel comfortable. A second species of flatfish, the dwarf flounder Achirus fasciatus, is very similar and the two are often muddled up. Fortunately, both species need the same things so it doesn't really matter which one you get. 

Moving up the size scale, the spotted garpike Lepisosteus oculatus is a big, very predatory fish that can only really be kept in giant aquaria. Spotted garpike are very widely distributed, occurring primarily in freshwaters throughout the United States east of the Rockies, though they are most common in the warm South Eastern states. Growing up to 60 cm (24 inches) in length, these fish do best kept in pairs or small groups and they do not like to be kept with aggressive or overactive tankmates, such as the more territorial cichlids. On the other hand, while they will eat any small fish they can catch, they are totally benign towards fish of similar size, and so can be mixed well with species like spotted sleeper gobies. They appreciate a planted tank, but failing that, judicious use of driftwood, bamboo, or rocks will work fine for creating regions of shade towards the back and corners of the tank. They do need plenty of open space though: thanks to their armour plating they are not agile fish and find it difficult to negotiate tight corners and thick tangles of branches or stonework. Several species of garpike occur in marine waters, but these tend to be the giant species, and the small spotted garpike is normally only found in either fresh or slightly brackish water. It will do well in freshwater aquaria or one with a specific gravity of anything up to 1.010.


Setting up the North American brackish water aquarium 

One way to create a nice North American themed aquarium would be to mimic a seagrass or turtle grass meadow. The hardier species of Vallisneria, such as Vallisneria americana and Vallisneria asiatica, are ideal for this purpose because they are fairly tolerant of low salinities. These plants grow rapidly when settled in provided a few basic requirements are met. As with all Vallisneria, strong lighting is essential, and depending on the depth of the tank, two or three fluorescent tubes the full length of the aquarium will probably be necessary. However, they aren't that bothered about substrate and plain gravel or sand can be used provided you regularly add an iron-rich fertiliser to the aquarium; the pelleted kind that you can place among the roots are especially useful for this. Vallisneria do not like be buried in the substrate, and the key to success with them is to make sure that the 'crown' part of the plant, where the roots and leaves grow from, is above the soil. 

I've had great success with Vallisneria using laterite-enriched pea gravel covered with a layer of fine silica sand, and practically all bottom-dwelling fish prefer a sandy substrate because it allows them to burrow and root about more easily. The only problem with using sand is that an undergravel filter cannot be used as well, and you do need to keep the gravel clean to prevent uneaten food from rotting and creating toxic gases. Surprisingly perhaps, the plants do a lot to help keep the substrate clean by transporting oxygen through their roots into the gravel and so promoting the growth of the 'good' bacteria that keep the aquarium healthy. 

Besides some sand and plants, a few smooth cobbles or stones make a great addition to this type of aquarium. A good idea is to confine the plants to one end of the tank, burying them in a sand bank propped up by stones. Place the nozzle or spray bar from the filter on this side of the tank so that the outgoing current of water pulls the long, strappy leaves across the top of the tank. This helps create the shade that so many of these fish appreciate without the need to actually fill the tank with rocks and bogwood. The rest of the bottom of the tank can be covered with a thin layer of sand, which will be much appreciated by fish like the freshwater flounder and violet goby which like to spend much of their time concealed for view. 

If the specific gravity is kept around 1.003 to 1.005, the aquarist will have no trouble keeping most of the fishes described here, though not all species will actually do well with one another (garpikes, for example, largely feed on livebearers and killifish). A North American-themed brackish water aquarium is an interesting change from the mangrove swamp paradigm that has been so popular over recent years, and has the added bonus of being as viable with a small aquarium as a jumbo one. Next time you make a visit to the South, take a look into some brackish water pools and streams, and see if you're not motivated to create a little bit of the bayou in your home!

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