Ask the WWM Crew
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Brackish water aquaria are a bit of a mystery to many people. While more popular than ever, thanks to the ever-increasing range of oddball fish being traded by enterprising retailers, there's still precious little about them in most fishkeeping guidebooks.
1. Must brackish water fish be kept in brackish water aquaria?
The answer to this depends on the species in question. Some supposedly brackish water fish are really freshwater fish that happen to be tolerant of salt. They can be kept perfectly well in freshwater aquaria. Such species include spiny eels, glassfish, wrestling halfbeaks, and (so long as the water is hard and alkaline) bumblebee gobies. Other species inhabit both brackish waters and the sea, and they can be kept in marine tanks. These include scats, monos, waspfish, green spotted puffers, and lionfish. Only relatively few absolutely must be kept in brackish water, most notably figure-8 puffers, knight gobies, and the orange and green Chromides.
2. What's the ideal salinity for a brackish water aquarium?
Salinity itself is next to impossible for hobbyists to measure directly, so instead we focus on the specific gravity, or density, of the water. Pure water has a specific gravity (SG) of 1.000, while regular seawater has an SG of around 1.025. A low-salinity brackish water tank, which would be ideal for things like gobies, livebearers, figure-8 puffers, and dwarf cichlids, needs an SG of about 1.005. The larger brackish water stuff, such as Colombian sharks, scats, monos, and so on, tend to prefer a higher specific gravity, with 1.010 being about right for long-term health.
3. How is specific gravity measured?
There are three ways to measure specific gravity -- you can use guesswork, a hydrometer, or a refractometer. Anything based on adding a certain weight or number of tablespoons of salt per gallon is basically guesswork. Because an opened box of salt absorbs water, it is impossible to reliably estimate how much salt is actually in a given quantity of the stuff. Hydrometers are much more reliable tools. There are two different designs, floating ones usually made from glass and resembling thermometers, and plastic ones containing an arm that points to the appropriate reading on a specific gravity scale engraved on the device. Refractometers are more expensive than hydrometers but are much more accurate. Advanced marine aquarists especially favour them for this reason, but they can also be used with brackish water aquaria, too. However, given that brackish water fish, by their very nature, have evolved to tolerate rapid changes in salinity, there's nothing wrong with sticking with an inexpensive hydrometer if you want to. Any small error in measurement will be well within the tolerances of your fish.
4. Must I use marine-grade salt mix?
Yes. As an interim measure, aquarium tonic salt is better than nothing, but over the long term your fish will be far healthier if you use proper marine mix. In addition, marine mix is cheaper than tonic salt and available in large boxes and tubs that make for even better value. It doesn't matter which brand you use, but be sure and follow the instructions and ensure that all the salt is dissolved before adding the brackish water to your aquarium
5. How do I mature a brackish water aquarium?
In tanks where the SG is 1.005 or less, the filter bacteria appear to be the same as those in freshwater tanks, so you can either convert an existing freshwater aquarium to brackish water or else 'seed' the filter in a new tank with filter media taken from a mature freshwater aquarium. Tanks with a specific gravity above 1.005 are most easily matured using either a fishless cycling method or by adding a few hardy fishes (black mollies are ideal).
6. Will plants grow in brackish water?
Provided the SG is around 1.003, then there are actually rather a lot of plants that will do well in brackish water. Most species that thrive in hard water also thrive in low-salinity brackish water, so species such as Echinodorus bleheri, Egeria densa, Ceratophyllum demersum, Crinum thaianum, Cryptocoryne wendtii, Hygrophila polysperma, and Vallisneria spiralis. There are even a few aquarium species that are actually brackish water specialists, being naturally found in such environments and consequently very good choices for such tanks where the SG is anything up to 1.005, even slightly higher. These include Bacopa monnieri, Crinum calamistratum, Cryptocoryne ciliata, Lilaeopsis brasiliensis, and Microsorium pteropus (Java fern).
7. Can I use bogwood in a brackish water aquarium?
Yes! Contrary to popular belief, bogwood is perfectly safe in a brackish water aquarium. Provided you are using proper marine salt mix, you will find that brackish water is sufficiently alkaline to neutralise any acidification caused by a few pieces of wood. If you are using a great deal of wood, then it is probably a good idea to monitor the pH and, if necessary, use an additional buffering agent such as coral sand in the aquarium.
8. What alternatives to plants are there for decorating a brackish water aquarium?
Brackish water tanks are most easily decorated in the same way as cichlid aquaria: with large pieces of rock and a few choice bits of bogwood. With species such as monos and scats, you will find that one or two giant rocks will look much more effective than a dozen smaller boulders, so choose your materials carefully. Since you aren't going to be using plants, you need to choose rocks that really work hard to evoke a rubble-strewn shoreline or harbour. Granite works exceptionally well, and can be procured easily from garden centres. What you don't want is Tufa rock; while it is safe to use in brackish water tanks, it looks rather bare and gets dirty rather easily. Instead of suggesting a natural habitat, your tank will instead look like a failed marine aquarium. Artificial mangrove roots are also available; though expensive, they look very good, and with a few plastic plants strategically siliconed onto them here and there, can be used to make a very convincing, zero-maintenance, mangrove-themed aquarium.
9. What's the best substrate for a brackish water aquarium?
If you're using an undergravel filter, then plain gravel with a certain amount of coral sand mixed in is perhaps the ideal because the coral sand will buffer the water quite effectively. In low salinity systems, add about one part coral sand to twenty parts of gravel, as you don't want the pH and hardness to go too high. In the higher salinity systems (where the specific gravity is 1.010 or more) then you can be more generous with the coral sand, and even a purely coral sand substrate will be perfectly acceptable. If you aren't using an undergravel filter, then simply choose whatever substrate suits your fish or plants. Burrowing and digging fish, such as gobies and soles, appreciate silica sand, for example, while plants often do best with fine, laterite-enriched, gravel.
10. Are brackish water fish good community fish?
Many, but not all, are very good community fish. Monos, scats, and Colombian sharks, for example, will all usually get along well with each other and with any other non-aggressive fish of similar size. Archerfish can be aggressive towards their own kind when kept in small groups, but are otherwise tolerant and easy to accommodate, and much the same thing can be said for brackish water cichlids such as orange Chromides and black-chin tilapia. However, some fishes are definitely not for the average community tank. Moray eels and pufferfish top this list, and while some aquarists have succeeded in keeping them with other species, this depends a lot on the situation and the temperament of the fish in question. It's essential to read-up on any potential purchase thoroughly before buying it in case it turns out to be aggressive, predatory, or both.
11. Are there any species to avoid?
Compared with the impossible-to-keep marines and the giant freshwater catfish, brackish water stock is, by and large, well chosen by importers and ideally suited to captive life. There are a few rare exceptions though. The most notable is the peacock puffer, Takifugu ocellatus, a subtropical species that only seems to last for a few weeks or months, even in the hands of expert fishkeepers. The mangrove jack, Lutjanus argentimaculatus, is another fish to avoid. Though not especially difficult to keep, its large adult size (up to 150 cm) makes it a poor choice for the confines of the home aquarium. Finally, the four-eyed fishes (Anableps spp.), though fascinating animals, are truly fish for experienced aquarists being relatively large schooling fish that have a reputation for being nervous and difficult to keep.
12. Is there truly a dwarf mono? What about a dwarf scat?
There is indeed a small species of mono, Monodactylus kottelati. It gets to an adult size of no more than 8 cm, making it an ideal aquarium fish. You won't see this fish traded under its Latin name though, as importers lump this fish with its larger cousin, Monodactylus argenteus, under the one common mono name. Telling them apart isn't too difficult though. Whereas M. argenteus has dorsal and anal fins of similar length, the anal fin on M. kottelati is much longer than the dorsal fin. In addition, M. kottelati also tends to have more orange on the dorsal fin, whereas M. argenteus is predominantly yellow. Unfortunately for aquarists, although there is a relatively small scat, Selenotoca papuensis, which only gets to about 9 cm in length, it isn't commonly traded, and even experts have trouble distinguishing in from the much larger Selenotoca multifasciata. About the only consistent difference that aquarists will be able to use is the coloration. Compared with S. multifasciata, S. papuensis has fewer, thicker bands and noticeably larger spots. A good rule of thumb is that if the black spots on the flanks are the same size as they eye, the fish is S. papuensis; if they're much smaller, it's S. multifasciata.
13. Why are 'freshwater' soles so difficult to keep?
Soles and flounders aren't especially delicate, but they are tricky to feed. Many specimens will only accept live foods such as bloodworms immediately after import, and only gradually can they be weaned onto dead foods or pellets. The very small (2.5 cm) specimens sometimes offered for sale are exceptionally difficult to look after without a regular supply of live food. Most soles are active at night, at least to begin with, so you need to provide food after the lights are out and also take care that there aren't any nocturnal fish, such as catfish, that might steal the food from them. Besides the problems with feeding, a few species are subtropical in origin, such as the New World species Trinectes maculatus and Catathyridium jenynsii, and these simply will not do well kept in a tropical aquarium.
14. Do all archerfish need brackish water?
A surprise to many people is the fact that most species of archerfish are actually freshwater fish, not brackish water ones. However, the fish offered for sale are almost always Toxotes chatareus and Toxotes jaculatrix, both of which need brackish water. The exact specific gravity doesn't matter, and anything from 1.005 to 1.010 will suit them fine.
15. Can I keep rainbowfish in the brackish water aquarium?
Most rainbowfish live exclusively in fresh water, and none of them are true brackish water specialists. Nonetheless, those robust species that enjoy hard, alkaline water, such as Melanotaenia boesemanni and Melanotaenia mccullochi, will also do very well in slightly brackish water. A specific gravity of up to 1.003 is the upper limit in most cases, which will also suit things like gobies and orange Chromides, as well as other salt-tolerant freshwater species such as glassfish and halfbeaks.
16. Are there any brackish water catfish?
Catfish are actually very common in brackish water habitats, but most of the species found in such places tend to be large and predatory, and hence are not widely traded. Among the brackish water specialists are the Colombian shark catfish, the estuarine Bagrid (Mystus gulio), and the large banjo catfish of the genera Aspredo and Platystacus. Adult eel catfish, Plotosus lineatus, thrive in brackish water even though as juveniles they are normally sold as marine fish. There are even some Suckermouth catfish that prefer brackish water, such as Hypostomus ventromaculatus, but most of the commonly traded 'plecs' will not do well in brackish water.
17. Can I keep invertebrates in a brackish water tank?
Another surprise to many aquarists is that many of the freshwater invertebrates they are familiar with are actually brackish water animals in the wild. While most of these will simply be viewed as food by large fish like monos and puffers, in the low salinity tanks favoured by things like gobies, a few invertebrates will fit in very well. Algae shrimps, Caridina japonica, do well in slightly brackish water and will obviously be very useful as general scavengers and algae eaters. Almost all the 'freshwater crabs' are brackish water species and can be an amusing novelty in tanks with some dry land for them to forage on. Another nice addition is the olive nerite, Vitta usnea, most often traded as Neritina reclivata. This attractively marked snail generally does not harm to plants but will help rid a tank of algae, particularly diatoms. It is brackish water snail in the wild, despite being quite long-lived in freshwater aquaria. The zebra nerite, Neritina natalensis, and the spiny periwinkles of the genus Pachymelania are also good choices for slightly brackish aquaria. The Malayan livebearing snail positively thrives in brackish water and makes an excellent addition to a tank with a sandy substrate, where it will eat up any leftover food or organic matter and thereby prevent any problems with anaerobic decay. The Colombian Ramshorn snail does well in slightly brackish water, but it is a notorious plant eater. Among the invertebrates that do badly in brackish water are apple snails, swan mussels, fan shrimps, and crayfish.
18. Do I use fresh or marine aquarium medications?
Always use medications formulated to work in both fresh and marine aquaria. That way, you're covered regardless of how high or low the salinity is in your aquarium.
19. Will carbon and zeolite work in brackish water?
Carbon works perfectly well in brackish water, but zeolite (ammonia remover) will not work at all. Salt draws ammonia out from the zeolite and back into the water, which obviously defeats the point of having the zeolite in the first place!
20. Where can I find out more?
Practical Fishkeeping regularly features articles on brackish water aquaria, and Aqualog and TFH publish books dedicated to the subject, both cleverly entitled Brackish Water Fishes. Most other freshwater aquarium books will have at least some coverage of the subject, but these tend to focus on the most familiar species like monos and scats, and very often the information is either debateable or just plain wrong. My own Brackish Water Aquarium FAQ is another useful source of information.