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The Salt-allergy Myth
One of the most persistent myths in fishkeeping is that catfish are allergic to salt. The rationalisation behind this is that because catfish lack scales (which is true) they are much more sensitive to the osmotic effect of brackish water (which is nonsense). Consider this: moray eels also lack scales, yet virtually all of them live in the sea, with only a handful being found in brackish or fresh water.

The ability of a fish to control the flow of salts and water in and out of its body depends not on whether it has scales or not, but on its overall physiology. This, in turn, depends on the how evolution has modified that species to fit into a particular environment. Catfish adapted to soft water habitats, for example, are indeed unlikely to to do well in a brackish water aquarium, but the same holds true for all the other soft water aquarium fish, such as barbs, tetras, loaches, gouramis, and so on. What might come as a surprise to many catfish enthusiasts is that there are lots of catfish adapted to brackish water environments, and there are two families that are primarily marine in distribution, containing relatively few species that inhabit completely freshwater environments. These are the families Ariidae and Plotosidae, known as the sea catfish and eel catfish respectively. A few other families, including the Aspredinidae, Bagridae, Clariidae, Ictaluridae, Siluridae, and Loricariidae, include species that either tolerate brackish water or are found primarily in brackish water, further shattering the myth of catfish as being allergic to salt.

Brackish water plecs
Easily the question I get asked most often is whether or not plecs can be kept in a brackish water aquarium. The short answer is probably not, but the long answer is a much more complex than that.

To begin with, there are species of loricariid catfish that are apparently adapted to slightly brackish water habitats. In aquaristic terms, that would correspond to a specific gravity, or SG, of around 1.003 to 1.005. Hypostomus ventromaculatus is one such species, being found primarily in the Amazon estuary and according to FishBase only within the tidal, brackish water region of the river. Another species, Hypostomus watwata, is also common in slightly brackish environments, even being found living among mangrove tree roots. While these fish sound like ideal additions to a low-salinity brackish aquarium alongside things like mollies, gobies, and dwarf cichlids, the problem is that they are not traded much, if at all

 

Hypostomus plecostomus

 

Hypostomus plecostomus is another species known to inhabit slightly brackish water, though in this case such observations have often been made outside of its natural range. In Florida, for example, where this fish has become widely established, it is quite commonly found in slightly brackish water.

Of course the main problem with using this species in the brackish water aquarium is that while the name Hypostomus plecostomus is commonly used in aquarium books and magazines to refer to the standard issue, common or garden plec, it is hardly ever the species available for sale. You are far more likely to find species of Liposarcus or Pterygoblichthys masquerading as "common plecs" in most aquarium stores.

Freshwater catfish that occur in brackish water
A number of catfish families include species that tolerate brackish water even if they don't need it. Siluris glanis, for example, one of the Siluridae, is normally found in freshwater rivers but can also be the slightly brackish water fringing the Baltic and Caspian seas. The same holds true for various species of clariid and ictaluriid catfish. These fish probably don't have any specific adaptations to brackish water, it's just that they are incredibly hardy and tolerant of sub-optimal conditions. Ictalurus punctatus in particular thrives in low salinity brackish water aquaria, though its large adult size and predatory nature do limit its usefulness.

Brackish water specialists

 

Mystus gulio

 

Catfish that tolerate brackish water are one thing, but species that actually prefer brackish water conditions are something else. Mystus gulio is one such species. It is a large bagrid catfish known in its natural range (South and South East Asia) as the estuarine catfish, a good indication of its preferred habitat. It is large (45 cm), silvery, schooling catfish that has only rarely been traded as an aquarium fish. Superficially similar in form and habit to the popular Colombian shark catfish (discussed below) it contrasts with that species in requiring slightly rather than strongly brackish water. It is adaptable though, and is common in completely freshwater rivers, and is even sometimes found in the sea. Nonetheless, for optimal health, this fish should be kept in slightly brackish water around the SG 1.005 mark. While not recommended for use alongside scats or monos, which prefer more strongly brackish conditions, this would be a fine companion for large sleeper gobies, green chromides, Siamese tigers (Datnioides spp.), and other large but peaceful, low salinity, brackish water fish.

It should be noted that Mystus gulio is unique among Mystus species in being a brackish water specialist; most of the other Mystus species sold as aquarium fish are strictly freshwater animals and should be maintained accordingly.

 

Bunocephalus coracoideus

 

Another group of catfish primarily associated with freshwater habitats but including a few brackish water specialists are the banjo catfish. The small species, like the ever-popular Bunocephalus coracoideus, are entirely freshwater fish, but it is the larger ones, such as Aspredinichthys, Aspredo, and Platystacus spp., that are the brackish water specialists.

These big banjos typically grow to around 20 to 30 cm, but are otherwise very similar to the dwarf species in terms of habits and requirements. In the wild they are normally found in coastal regions, though not always in brackish water, but they are not found in the sea. Like Mystus gulio, a low salinity suits them best, although they can be maintained in freshwater aquaria too, provided the water is not too soft or acid.

Marine catfish
When it comes to salty water, two catfish families stand out: the Ariidae and the Plotosidae. The Ariidae is perhaps the best known to freshwater fishkeepers, with one species, Ariopsis seemannii, being very widely sold as the Colombian shark catfish.