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Brackish without the salt

Do all 'brackish water fish' really need brackish water?

by Neale Monks


One of the commonest questions in my PFK post-bag is 'does this fish need brackish water'. All aquarists agree that scats, monos, and archerfish need brackish water, but when it comes to thinks like kribensis, spiny eels, and bumblebee gobies the facts are much less clear. Some books and web sites will insist that these species need brackish water, while others just as adamantly hold the opposite view. What's going on here? Surely, if a fish is found in brackish water, it must need brackish water, and if its not, then it doesn't. As with many things in life, it's just not that simple. 

For one thing, there's no such thing as 'brackish water'. In contrast to seawater or the black waters of the Amazon, brackish water isn't one specific set of water conditions but a whole spectrum. By definition, brackish waters occur where freshwater merges with the sea and in many places this takes place over hundreds of miles. High up an estuary, the salinity is very low and the aquatic animals and plants will be mostly freshwater types with only a few interlopers from the sea. Consider our own River Thames. Where the tidal flow ends at Teddington, hardy cyprinids like roach and bream that are common, while freshwater-tolerant marine species like flounders and sea bass are only occasionally found. By contrast, once the Thames rolls into London and becomes more saline, the bream and roach are gradually replaced by eels, sea bass, flounders, and plaice. The Thames Estuary proper, sandwiched between Essex and Kent, is so salty that it is entirely lacking in freshwater fish apart from migratory species like salmon and shad able to move between the river and the open sea. 

What holds for the Thames holds for every other river in the world. At one end, you have freshwater fish able to tolerant salty water, while at the other end you have marine fish that can survive in water with less salt than the sea. It's no surprise that among the species sold as brackish water fish include some fish belonging to freshwater groups, like the Cichlidae, while others are members of marine groups like the Gobiidae. However, if a fish is found in brackish water, does that mean that it needs to be kept in brackish water? Moreover, could brackish water do some brackish water fish harm over the long term? In this article, we'll look at those aquarium fish that are considered brackish water by some experts but fresh water species by others. 

Figure-8 Puffer, Tetraodon biocellatus 

The figure-8 puffer is a small (to 8 cm) attractively marked pufferfish from South East Asia. The specimens offered for sale to aquarists appear to be collected from freshwater streams and rivers in Indonesia, and they are invariably shipped and sold as freshwater fish. Nonetheless, over the years, many aquarists have reported that they do very well when kept in slightly brackish water, and in many cases seem to be less disease prone and certainly longer lived than specimens kept in freshwater aquaria. On the other hand, it cannot survive in seawater, which contrasts sharply with brackish water puffers like Tetraodon fluviatilis and Chelonodon patoca, which do very well in salt-water aquaria. 

One explanation is that the fish that are collected are invariably juveniles, and like many other fishes it is likely that they while they breed in fresh water the adults actually live in brackish water. Given that they do not do well in strongly brackish or marine conditions, it seems probably that they inhabit the upper part of estuaries, where the vegetation is thickest, and it can find plenty of its preferred prey, small snails and insect larvae. 

Received Wisdom:                Keep in fresh water.

PFK Recommendation:         Best kept in slightly brackish water; use a specific gravity of around 1.005. 

Peacock Spiny Eel, Macrognathus siamensis 

Spiny eels are popular aquarium fish, and many aquarists have considered adding a specimen or two to their community aquarium. While all are predatory, the smallest species, like the peacock spiny eel, are perfectly safe with fish too big to swallow, such as gouramis, barbs, Corydoras, and so on. Many are attractively patterned as well, and once settled in are not at all shy. The peacock spiny eel is perhaps the most widely traded, though often under erroneous Latin names such as Macrognathus pancalus and Macrognathus aculeatus, names that actually belong to similar but distinct species. The peacock spiny eel is a pinkish-brown fish that has a distinctive cream-coloured stripe running from the eye along the flanks to the base of the tail. On the dorsal fin close to the tail, there is a row of dark eyespots, and it is from these that it gets its common name. 

Spiny eels are primarily found in fresh water, with slightly acidic to neutral water conditions probably suiting them best; a few species also occur in slightly brackish water, although the peacock spiny eel is not one of them. Wherever they are found, spiny eels stay close to the substrate, although in heavily planted aquaria, they will hide among the leaves as well. Their fondness for digging can be their undoing, and in tanks with gravel instead of sand, it is common for these fish to scratch their sensitive skins while trying to push their way into the substrate. Injured fish are vulnerable to bacterial infections, and it is for this reason that spiny eels have acquired a reputation for being delicate and difficult to keep. 

Adding salt to an aquarium is certainly one way to inhibit external fungal and bacterial infections on spiny eels, and quite probably, this is where the idea that they are brackish water fish came from. In the right aquarium, one with a sandy substrate, such precautions are unnecessary. Almost certainly, the reason these fish so rarely breed in captivity is that aquarists have been keeping them in slightly brackish water instead of the soft, acid water they prefer. 

Received Wisdom:                Keep in slightly brackish water.

PFK Recommendation:         Salt is unnecessary if this fish is kept well. 

Bumblebee Gobies, Brachygobius spp. 

Bumblebee gobies are probably the most popular gobies available to aquarists, and with good reason. They are small, colourful, hardy, and surprisingly long-lived (some have lived for over four years!). Despite their ubiquity though, there is some argument over their optimal water conditions. The facts are straightforward enough: in the wild, these fish are most common in fresh, not brackish, water. Some populations occur in water that is acidic and has virtually no measurable hardness; in other words, the kinds of environments discus and tetras enjoy. However, in captivity they breed most readily in slightly brackish water, and it is certainly possible to adapt adults to brackish water with a salt concentration as much as half that of sea water. Are these freshwater fish or brackish water ones? 

From my own experience, I have found bumblebee gobies to do well in both slightly brackish water and in hard, alkaline water. When kept in soft, acidic water, by contrast, they are much less active and more prone to bacterial and fungal infections, a common reaction among fish maintained are the wrong set of water conditions. It is hard to imagine that bumblebee gobies are migratory, swimming between the sea and freshwater, so it doesn't seem likely that they are like scats or monos, living in freshwater as juveniles but salt water as adults. More likely, certain populations of gobies are adapted to particular water conditions, and only specimens collected from soft and acid water in the wild are going to do well under such conditions in captivity. Some shipments of gobies seem to thrive in hard, alkaline water while others don't do so well, but all seem to prosper when kept in brackish water at a specific gravity of 1.003 to 1.005. 

In short, provided you have a high level of hardness and maintain the pH above 7.0, it is certainly possible for bumblebee gobies to do well in freshwater aquaria, but if your gobies seem off-colour or sickly, then adding some salt may be the thing to do. On the other hand, there's no reason not to keep them in a brackish water aquarium, and that is certainly the no-risk option. 

Received Wisdom:                Keep in brackish water.

PFK Recommendation:         If in doubt, keep in brackish water at a specific gravity of 1.005. 

Kribensis, Pelvicachromis pulcher 

Kribs are hardy, easy to keep dwarf cichlids from the Niger Delta region of West Africa. Among the easiest cichlids to breed, many aquarists have been surprised by their Kribs willingness to breed in even a busy community tank, not to mention their zealousness at protecting the fry from potential predators. Both sexes are gorgeous, brightly coloured fish, with the female perhaps taking the nod as the prettier of the two. They are also well known for being salt-tolerant cichlids, and they do indeed thrive in aquarium maintained at relatively low salinities (a specific gravity of 1.005 being perhaps the upper limit); but are they truly brackish water fish? 

Part of the answer must come from the fact that sex ratios in broods of Kribs raised in brackish water or hard, alkaline freshwater invariably contain a preponderance of males. When kept in soft and acidic water conditions, there are usually many more females than males. Given that water chemistry clearly influences the sex ratio within the brood, we can assume that the optimal conditions would be those resulting in equal numbers of males and females, and this is what you get when Kribs are kept at a neutral pH and in only moderately hard water. Since sea salt increases hardness and raises pH, if you want to breed these fish, don't keep them in brackish water. 

Received Wisdom:                Keep in slightly brackish water.

PFK Recommendation:         Best kept in neutral, slightly soft to moderately hard water. 

Black-chin tilapia, Sarotherodon melanotheron melanotheron 

While tilapias haven't generally become popular aquarium fish, one species that is sought out by aquarists is the black-chin tilapia. Unlike many of the other tilapias, this species is a paternal mouthbrooder. The females lay the eggs into a spawning pit, and then the male fertilises them and then gathers them up in his mouth. He will brood the eggs for two to three weeks, after which time the young fish are released and left to fend for themselves. 

Black-chin tilapias are invariably sold as freshwater fish, and they can certainly live and breed quite happily in fresh water. However, in the wild these fish are usually found in brackish water habitats such as mangrove swamps and coastal lagoons. They are also able to live and breed in seawater; this has allowed them to become established in the coastal ditches and canals of Florida and shallow water harbours in Hawaii. In Ghana, where these fish originally came from, black-chin tilapias are rarely found outside of brackish water habitats. Other, truly freshwater species of tilapia replace the black-chins once the salinity drops below a certain point, suggesting that black-chin tilapia is a brackish water specialist optimised for living in environments where the salinity varies. 

Does keeping them in fresh water do any harm? Tank-bred stock raised in fresh water across a number of generations don't seem to be particularly bothered, but some aquarists have noticed that the biggest, most colourful specimens are invariably ones raised in slightly brackish water. Since these fish aren't especially aggressive, they can make a good addition to a brackish water aquarium alongside robust species such as scats and monos. 

Received Wisdom:                Keep in fresh water.

PFK Recommendation:         Bigger and is more brightly coloured when kept in brackish water. 

Indian Glassfish, Pseudambassis ranga 

Perhaps more than any other tropical fish, glassfish have laboured under much confusion as to their preferred water conditions (see PFK, January 2006). To cut a long story short, the species of glassfish traded as aquarium fish normally inhabit fresh, not brackish, waters and do perfectly well when kept in neutral freshwater aquaria. Some species even thrive in soft and acidic water conditions, among them the Indian glassfish, Pseudambassis ranga

So why have them been invariably labelled as brackish water fish? There are glassfish in brackish waters, and it is entirely possible that misidentifications in the early years of the aquarium hobby resulting in ecological information from a brackish water species being attached to one of the freshwater species. They are also notoriously meticulous feeders, and a clear advantage of brackish water is that it allows the aquarist to feed live brine shrimps without worrying about the shrimps dying before they are eaten. Finally, brackish water can have a tonic effect even on freshwater fish, and since these fish can be a bit delicate upon import, slightly brackish conditions can minimise losses. 

Having said all this, there's no compelling reason for the aquarist to keep them in brackish water. While they can be kept in slightly brackish water should you need to, most glassfish will do as well, if not better, in freshwater conditions. 

Received Wisdom:                Keep in brackish water.

PFK Recommendation:         Does best in neutral freshwater. 

Mollies, Poecilia spp. 

At least five species of molly have been traded as aquarium fish, but the commercial stock is hybridised to such a degree that it is virtually impossible to describe any of the mollies offered for sale as purebred examples of one particular species. Most hobbyists have shown little interest in keeping 'wild type' mollies, with the result that commercial breeders focus exclusively on producing artificial varieties: black mollies, orange Sailfins, balloon mollies, an so on. These are all hybrids and understanding the biology of their ancestors has only a tangential relationship to what these fish need to thrive in home aquaria. 

About the only certainty that all aquarists can agree on is that mollies don't do well in soft, acid water. One persistent problem among mollies kept in such conditions is fungus on the fins and body, and a standard treatment for this has been to dip them in salt water at the first sign of trouble. Some aquarists go so far as to add salt to the tank prophylactically, effectively keeping them in a low-salinity brackish water aquarium. Some fish, notably Corydoras and tetras, are very intolerant of salt, so this cannot be done indiscriminately, but there's no question that mollies kept in slightly brackish water are much less disease-prone than ones kept in freshwater tanks. 

Does this make them brackish water fish? In the wild, mollies are very common in brackish water ditches and streams, though many populations are completely confined to freshwater habitats. Even though we can't say exactly what the genetic make-up of any given fish is likely to be, the odds are good that some of their genes come from brackish water ancestors. Many people have kept mollies without any problems in completely fresh water, but if you want to minimise the chances of fungus or fin-rot, adding at least a bit of salt seems the way to go. A specific gravity of 1.003 is more than adequate, and will be tolerated by many plants as well as numerous types of aquarium fish including other livebearers, rainbowfish, and halfbeaks. Bump the salinity up a little, and you can mix mollies perfectly well with brackish water fish including orange Chromides, gobies, and killifish. 

Received Wisdom:                Fresh or brackish water.

PFK Recommendation:         Though they can do well in hard, alkaline freshwater, mollies really thrive in brackish water conditions. 

Siamese Tigers, Datnoides spp. 

Siamese tiger fish are impressive predatory fish from South East Asia, and they are often included in shipments of oddballs alongside scats, monos, and puffers. This has led to them almost universally being classified as brackish water fish. However, of the five described species of Datnoides, no less than three of them are strictly freshwater residents and shouldn't be kept in brackish water over the long term. These include the most commonly traded species, Datnoides microlepis. The remaining freshwater species are Datnoides pulcher and Datnoides undecimradiatus, but these aren't traded as aquarium fish, so needn't detain us further. 

The two brackish water species are Datnoides quadrifasciatus and Datnoides campbelli, both of which are (albeit rarely) offered to aquarists. These fish need a brackish water aquarium with a specific gravity of around 1.005 to 1.010. Both can be maintained in freshwater aquaria for a while, but over the long term, brackish water is essential. If you are tempted to buy a Siamese tiger fish, it is therefore critical that you identify the species correctly and set up the tank appropriately. While Datnoides campbelli is easy to recognise, thanks to its brown-on-yellow colouration (and its steep price tag!), separating Datnoides quadrifasciatus from Datnoides microlepis is very difficult. 

Contrary to popular belief, the number of vertical bars on the flanks is not a reliable indicator. While it is true that many Datnoides microlepis only have three complete bars (the first through the pectoral fin, and then two more from the dorsal to the anal fins), some specimens have four complete bars, and so resemble Datnoides quadrifasciatus, which invariably have four of these bars. Additional characteristics to look out for are the shape (Datnoides microlepis is more deep-bodied); colour (Datnoides microlepis tends to be silvery while Datnoides quadrifasciatus is often yellowy or grey); and the size of the scales on the flanks (those on Datnoides microlepis are much smaller than those on Datnoides quadrifasciatus). 

Received Wisdom:                Keep in brackish water.

PFK Recommendation:         The commonest species does best in fresh, not brackish, water, so identify your fish carefully!

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