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.
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
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
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
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
make brackish water in a bucket and only then add it to the aquarium.
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
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,
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
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
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
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
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.
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
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.
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
the Virginia nerite (Neritina
and the zebra nerite (Puperita
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
also do well in brackish water, up to about half-strength seawater (SG 1.010 or
less). The less frequently traded West African snail
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
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,
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
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
but these are very, very rarely traded.
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
and these are
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
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
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.
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.
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).
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