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The Brackish Water Aquarium
21 March 2017
Brackish water occurs wherever rivers meet the sea and has a salinity in between freshwater and seawater. Estuaries and mangroves are the two best known brackish water habitats, and both yield many different types of aquarium fish. Because they combine elements of both freshwater ecosystems and marine ones, the diversity of fish found in these places has things in common with both. So while there are brackish water catfish, cichlids, and livebearers related to those found in freshwater habitats, there are also species that have come from the sea, such as pufferfish, gobies, and eels. It is the wonderful variety of brackish water fishes that makes brackish water aquaria so popular.
Making brackish water
Setting up a brackish water aquarium is not difficult. In most regards it is identical to a freshwater tank. The key difference is the water chemistry. Brackish water needs to be hard, alkaline, and contain about 10% to 50% the salt of normal seawater. Because salinity is difficult to measure directly, aquarists instead use a device called a hydrometer to measure the density of the water. The density of pure water, also known as specific gravity, is 1.000 whereas that of normal seawater at tropical temperatures is about 1.025. Brackish water is typically maintained at a specific gravity of 1.003 to 1.010, depending on the species being kept.
Proper marine aquarium salt mix should be used. While aquarium tonic salt will do as a stop-gap, over the long term the fish will be healthier if marine salt is used. Marine salt doesn't just contain sodium chloride but also a host of other mineral salts. These help to buffer the water, keeping it hard and alkaline. Most brackish water species need a pH of around 7.5-8 and a hardness of at least 20-degrees GH.
Decorating the brackish water aquarium
Brackish water does not suit the majority of aquarium plants, so it is easiest to rely on rocks, bogwood, and sea shells for decoration. An effective display can be created by using few large pieces of rock to form an imposing, reef-like structure around which the fish will swim. The bottom of the tank can then be covered with a mixture of silica sand and coral sand to a depth of 2-3 cm. This will provide gobies and flatfish with a place to dig. A few smaller stones and seashells can then be scattered about to complete the scene. Gobies and dwarf cichlids in particular appreciate seashells and will use them as places to rest and spawn.
Bogwood can be very effective in brackish water aquaria, but it does
tint the water over time and can acidify the water as well. One or two
pieces shouldn't make much difference, but if you use a large amount for the purposes of re-creating a mangrove swamp look, monitor the pH carefully and if necessary add a high-pH buffer with each water change.
Most of the ceramic and plastic ornaments designed for use in aquaria are safe in salt water, and these can be used very effectively in brackish water aquaria. Ornaments in the forms of caves, tree roots, and shells are especially effective, but some aquarists use things like chains and Roman pots to re-create a harbour-like look.
Salt-tolerant aquarium plants
Although most aquarium plants do not do well in brackish water, there are a surprising number that will thrive at a specific gravity of 1.003. Most species of aquarium plant that will do well in hard, alkaline water will do fine at this level of saltiness, including Cabomba caroliniana, Crinum thaianum, Cryptocoryne wendtii, Echinodous bleheri, Egeria densa, hornwort, and most species of Vallisneria.
Even at higher salinities, there are a few brackish water specialists
quite widely traded and easily obtained. In tanks with a specific
gravity of up to 1.005, you can use Bacopa monnieri, Crinum calamistratum, Crinum pedunculatum, Cryptocoryne ciliata, Java fern, Java moss, Lilaeopsis brasiliensis, and Samolus valerandi.
Popular brackish water fish
Figure-8 puffer: This small (8 cm) species is active and brightly coloured, and is consequently among the most popular of all the puffers. It is not especially aggressive, but some specimens can be persistent fin-nippers, and so cannot be recommended unreservedly for community tanks. Livebearers such as guppies are particularly at risk, and if overcrowded, figure-8s will fight with one another. On the other hand, a single specimen will usually mix well with small, sturdy, low-salinity brackish water fish including bumblebee gobies, orange chromides, and glassfish. Figure-8 puffers only need a low specific gravity, ideally 1.005.
Ceylon puffer and green spotted puffer: These two species are often confused. Ceylon puffers tend to have saddle-like markings on the back, whereas green spotted puffers has small black spots against a light green background. Names aside, these fish are essentially identical in terms of upkeep, being relatively large (around 15 cm) when mature and far too aggressive to be kept with tankmates beyond their own kind. They are territorial, and when keeping more than one specimen, allow about 100 litres per fish and make sure the aquarium has lots of hiding places.
Scats: These are wonderfully active, attractively patterned fish that work well in large community tanks. Most species are rather large (typically around 20 cm in captivity), so these are really fish for aquaria with a capacity of 200 litres or more. On the other hand, they are incredibly hardy, and will eat almost anything, both animal and vegetable. Feed these fish a mixed diet, preferably with plenty of greenstuffs and algae. Single specimens can be kept, but these fish look best in groups. There are various species of scat, but all need a specific gravity of around 1.010 to do well over the long term.
Monos: Ideal companions for scats, monos are even more energetic. The common mono is a circular fish with yellow tips to its triangular dorsal and anal fins. It grows to about 15 cm in home aquaria and needs plenty of swimming space. The West African mono is of similar size but taller, being rather like a freshwater angelfish in shape and colour. A dwarf species of mono is sometimes offered for sale as the common mono. It differs from the common mono in having orange rather than yellow markings and the anal fin is distinctly deeper than the dorsal fin is tall. It only grows to about 8 cm in length. All monos need at least moderately brackish water; a specific gravity of 1.010 is ideal. Monos are easy to feed, and will take floating pellets, flake, and all kinds of frozen, meaty foods. They are schooling fish, and do best kept in groups of three or more.
Colombian shark catfish: These peaceful, predatory catfish like to be kept in groups, something that can be problematical given their large adult size (expect at least 30 cm). While they will eat small fish, they get along well with large species including scats and monos. Feeding presents no problems, as they will greedily take catfish pellets as well as frozen prawns, mussels, and other meaty foods.
Archerfish: Although there are freshwater archerfish, most of the specimens sold in aquarium shops are brackish water species. In terms of care they are identical to the monos described above. Archers can be trained to spit in aquaria, most easily by sticking small bits of food (such as frozen prawn) onto the glass above the waterline. Archerfish are quite big, and will grow to at least 15 cm in captivity. They are also highly piscivorous, and will eat any small fish they can catch.
Knight gobies: These medium-sized, predatory gobies are much appreciated for their fine colour and active habits. Males can be very territorial, so they should not be overcrowded. Will eat most live foods as well as frozen bloodworms and, eventually, flake food. Note that these fish will also eat smaller fish, including small gobies and livebearers. Does best at a specific gravity of around 1.005.
Bumblebee gobies: In terms of care, similar to the knight goby, but needing proportionately smaller foods. Frozen bloodworms make a useful staple, but these fish also appreciate live brine shrimp and daphnia. A fun fish for a small aquarium, they are most fun in groups, with each fish having a shell or cave, with the tank stocked at around one goby per 5 litres of water. Mixes well with peaceful, mid- and upper-level fish such as livebearers.
Orange chromides: These pretty dwarf cichlids are not at all disruptive and make a good addition to the mixed community tank. Provide each fish with its own cave to avoid fights, and with luck, you'll be able to see them breed. The eggs and fry are small and quite tricky to raise. Another low salinity species, keep orange chromides at a specific gravity of 1.005.
Livebearers: Mollies thrive in brackish water and make excellent companions for non-predatory brackish water species. Sailfin mollies in particular can add a splash of colour to the aquarium as well as being useful algae eaters. Wrestling halfbeaks are unusual livebearers for the low specific gravity aquarium (at 1.005 or less). Needle-shaped and very feisty, these fish grow to around 7 cm in length and produce small broods of surprisingly large fry. Halfbeaks are notorious jumpers, and must be kept in a tank with a hood!