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I am writing this article in considerable pain. Not because I don't like writing, but because I have just fallen onto an oyster bed while pursuing pufferfish and scratched myself to pieces. As might be clear, I'm not in the UK right now, but at a friend's holiday home in Florida. A few meters down the street she lives on is a small mangrove swamp bordered by an artificial reef that forms the entrance to the marina. The reef is covered with oysters and barnacles, and while it is only a few meters long, the abundance of molluscs and crustaceans makes it a prime pufferfish feeding ground. Dozens of Sphaeroides maculatus can be seen gliding across the rocks searching for crabs, shrimps, and small molluscs that haven't hidden themselves in crevices adequately enough. This particular species resembles the Asian figure-eight puffer in appearance, though it is quite a bit bigger, the average specimen being about 15-20 cm in length. Catching one of these puffers for the aquarium is much more difficult than spotting them though, and while pursuing one specimen with my net I managed to miss my footing and fall onto the rocks.
While this event signals the end of my fishing expedition on this particular day, it has made me think about the puffers, their environment, and the other fishes swimming about the reef with them. For a start, the salinity here fluctuates with the tide, from about half to full strength sea water. The vegetation consists of a mixture of mangroves and salt tolerant marsh species such as various sedges and reeds. Some of the fish species are ones you would normally associate with freshwater habitats such as garpike, Lepisosteus spp. Then there are those species that we more frequently associate with brackish water habitats, like mullets, snook, and ariid catfish, Mugil cephalus, Centropomus undecimalis, and Arius felis being especially common here. Finally, there are the fish that normally inhabit marine conditions but will enter brackish waters if the mood takes them. The pufferfish are just one example of these fish, in this case to take advantage of the rich shellfish hunting grounds that exist on oyster beds. Another species that does much the same thing are sauries, large marine versions of the more familiar halfbeaks, attracted here by the schools of small fish, in particular juvenile marine species, that look for safety among the rocks and mangrove roots. Various species of snapper, jack, and porgy hunt here as well, and in turn provide good sport for anglers too.
So what does this tell the aquarist with an interest in brackish water fish? Above all else, brackish water habitats blend marine and fresh waters, and this is reflected in the fact that the species of fish that live there are not just freshwater forms tolerant of salt, but also marine species that will put up with reduced salinity. In other words, the aquarist can look in both the freshwater and the marine sections of his or her local tropical fish store when looking for species for the brackish water aquarium. But having said that, marine fish are notoriously difficult to keep compared with their generally quite adaptable freshwater cousins. So which species are the best bet for the ambitious aquarist looking to add something from the sea to a collection of scats, monos, or Colombian shark catfish?
Top of my list of marine species for the brackish water aquarium are two Asian pufferfish, the dog faced puffer, Arothron hispidus, and the milk spotted puffer, Chelonodon patoca. Both are relatively peaceful (by pufferfish standards at least), hardy, and in the case of the dog faced puffer, quite commonly traded and inexpensive. While I was a student at Aberdeen University, I used to take trips to neighbouring towns and cities to find stock for the two 200-gallon aquaria I helped to maintain in the Department of Zoology. On one particular trip to a store in Dundee I came across a tank containing a bunch of attractively patterned pufferfish. They were being kept in the slightly brackish water along with some more familiar fish, but what immediately caught my attention about them was that unlike the usual fresh and brackish water puffers, these fish were not some variation on the theme of 'green and spotted', but brown, with a creamy white underbelly and a nice pattern of grey stripes and spots across the flanks.
I took the chance with these fish only because I knew I had two large tanks to keep them in and all my other fish were robust, hardy species. It's all too easy to end up with fish that don't fit in well with your other species, but in this case I lucked out, and reference to a marine fish encyclopaedia revealed that I had bough two juvenile dog faced puffers. Normally sold in the marine section of tropical fish stores, for whatever reason this batch had been exported as freshwater fish, but seemed to be doing well nonetheless.
While they will tolerate low salinities for a while, the best way to keep dog faced puffers is in an aquarium with a specific gravity of 1.010 or above. Such conditions suit scats and monos very well, and these puffers are usually peaceful towards species other than their own. Marine aquarists often keep them with hardy marines like lionfish and moray eels in fish-only aquaria, and like those species they are not particularly sensitive to things like high nitrate levels, making them great species for less experienced aquarists. In the brackish water tank the main thing is to ensure that the pH and water hardness stay high. Using a good quality marine salt mix will go a long way to taking care of this, but play it safe by incorporating calcareous materials like coral sand in the aquarium as well. Although not widely traded, there are some other Arothron species that can be used in brackish water systems, most notably Arothron reticularis, sometimes sold as the stars and stripes puffer. On the other hand, neither Arothron meleagris nor Arothron nigropunctatus will do well in brackish water, which is a shame as these are very pretty and quite frequently traded species
Chelonodon patoca, known in the hobby as the milk-spotted puffer, is usually sold as a marine fish but sometimes turns up as freshwater species as well, and this is a good clue to its overall hardiness. Like Arothron hispidus, in the confines of the aquarium it tends to be aggressive towards its own species, but is otherwise an equally good community fish. Both the milk spotted puffer and the dog faced puffer are hardy and tolerant fish, and compared with most other marine fish these aren't going to keel over and die at the first sniff of a nitrite ion. On the other hand, this isn't a licence for bad aquarium management, and if you want these fish to do well, good filtration and oxygenation are essential.
Damselfish are primarily marine in occurrence, but a few do venture into brackish, even fresh, waters. One of the most commonly found in shallow coastal waters other than coral reefs is the sergeant major, Abudefduf saxatilis, and provided the specific gravity is kept above 1.015, these fish will settle into high salinity brackish water aquaria quite well. In captivity it is often rather territorial towards other damsels, and so is best kept alone, but it does have character and readily becomes tame enough to be hand fed. As such, it makes a great personality fish to combine with active, schooling species like monos.
A few species of damselfish are truly euryhaline, and these occur often enough in freshwater to have been traded as 'freshwater damsels' or 'freshwater gregories'. These include Neopomacentrus taeniurus, Pomacentrus taeniometopon, and Stegastes otophorus. None are terribly commonly seen, but at least one of them, Pomacentrus taeniometopon, quite often gets mixed up with the black neon damsel Neoglyphidiodon (Abudefduf) oxyodon. Unlike the true black neon, Pomacentrus taeniometopon lacks the white, saddle-like stripe running up from the flanks and over the back. Dedicated brackish water aquarists should definitely keep their eyes open for this mix-up, and shop accordingly. As with the sergeant major, this species is best kept either alone or in matched pairs, but it will otherwise get on well with non-predatory brackish water species such as monos and scats just fine.
At least one species of brackish water catfish has become very popular in recent years, the Colombian shark catfish Hexanematichthys seemanni. Another catfish that brackish water aquarists might want to consider is the striped eel catfish, Plotosus lineatus, that is more normally kept by marine fishkeepers. Like the shark catfish, this is a big, predatory species that cannot be kept with substantially smaller species, but provided you make that allowance when choosing its tankmates, it is otherwise a peaceful and reliable community fish.
Juveniles are very sociable, and really need to be kept in fair sized shoals, but as the fish mature, they become more independent, and adults are more often found alone or in groups of two or three. Young striped catfish need to be maintained in fully marine conditions, but once these fish get to around 10 cm or so in length, they become tolerant of reduced salinities, and they can be adapted to anything down to a specific gravity of around 1.012. Given that these fish reach a size of 30 cm or so, they will need a large aquarium, but their dark, tadpole-like shape makes for an interesting fish that contrasts nicely with more brightly coloured species like scats and monos. Like most other catfish, they are opportunistic feeders, and while they will need regular feedings of shrimps and other shellfish, they will also do double duty as scavengers, taking any flake or pellets the other fish miss. Do be sure and give each catfish its own cave though, as these fish will only settle in if they feel secure. One final thing to remember: like Colombian shark catfish, these fish have venomous spines in their fins, and by all accounts their sting is excruciatingly painful. Anyone stung by these catfish should seek medical attention at once.
Though one species of snapper, the mangrove jack Lutjanus argentimaculatus, is sometimes sold as a freshwater or brackish water aquarium fish, marine aquarists are generally much more familiar with this group of active, predatory fish. However, many species do range into the brackish waters around mangroves and estuaries from time to time, though only one of these, the emperor snapper Lutjanus sebae, is of much interest to the aquarist. It is fairly widely traded on account of the handsome patterns that juvenile fish sport, and like the dog faced puffer has a reputation for being easy to maintain and generally hardy. But there are some downsides to this fish, the chief of which is that it is a big species, commonly exceeding lengths of 45 cm even in captivity. It is also boisterous and predatory, and so cannot be kept with timid fish (like archerfish or batfish) or small ones (like gobies and livebearers). Finally, while robust, this fish does need an aquarium with a decent filter as its prodigious appetite tends to make it difficult for the aquarist to keep ammonium and nitrite levels low. All in all, not a species that can be recommended unreservedly, but possibly something for the advanced aquarist. Like most of the other species discussed here, this fish likes brackish water at the saltier end of the scale, a specific gravity of 1.012 or more being ideal.
Scorpionfish are another group that marine aquarists are much more familiar with than fishkeepers keeping freshwater or brackish water species. Only one scorpionfish is regularly traded as freshwater fish, the bullrout or freshwater lionfish, Notesthes robusta. But several of the marine species make occasional sojourns into brackish waters, including the quite commonly traded dwarf lionfish, Dendrochirus brachypterus. It is quite an attractive beast in a sombre sort of way, with a handsome mixture of cream, red, and brown stripes and spots. In the wild this fish occasionally turns up in high salinity brackish waters, particularly in estuaries and tidepools, and in captivity will do well in aquaria with a specific gravity of 1.015 or more. Because this species stays relatively small (around 10 cm or so) and doesn't swim about much, it is a good fish for a middle sized aquarium, and will mix very well with other peaceful fish including scats and monos. While it could also be kept with adult sailfin mollies, for example, it is highly predatory and will readily eat any fry they produce. A final issue that needs to be borne in mind is that these fish have venomous spines, and needless to say, these fish must be handled very carefully.
The stripey, Microcanthus strigatus, can easily be mistaken for a scat, but whereas the scats have stripes that run from the top of the fish downwards, the stripes on this fish start roughly at its forehead and run diagonally downwards towards the anal fin. The result is a neatly patterned and attractive fish that for whatever reason isn't nearly as popular in the fishkeeping hobby as it should be.
The stripey doesn't grow quite as large as the typical scat, around 15 cm or so in captivity, making it much easier to keep this fish in groups without overloading the filter. But in other regards it is much the same fish: very hardy, ready to eat all sorts of fresh and prepared foods, and completely peaceful towards its tankmates. However, the stripey is not nearly as euryhaline as the Scatophagidae, and is unable to swim freely between fresh and salt water. It must be kept in systems where the salinity fluctuates between about two-thirds and full-strength sea water. A constant salinity isn't necessary, these fish will handle fluctuations just fine, you just need to make sure the salinity doesn't drop too low.
My last suggestion for the advanced or ambitious brackish water fishkeeper is the orbiculate batfish Platax orbicularis. While widely sold to marine aquarists for their reef tanks, juvenile orbiculate batfish are usually found in estuaries and around mangroves rather than coral reefs. This is reflected in their shape and colouration, which closely resembles a dead leaf. In the wild, this allows them to pretend to be a dead leaf if a predator swims by, even down to leaning over to one side and drifting, just as a real leaf floating downstream might. Only once they mature do these fish start forming schools and then move out onto the coral reefs.
Like the stripey, orbiculate batfish aren't for the low salinity brackish water aquarium, but if you have a large tank with a specific gravity of 1.012 or more, then this species can make good specimen fish. But you need a really big tank; these fish grow very quickly, and even in captivity easily exceed 50 cm in length (and like freshwater angelfish, they're even taller than they are long!). Like scats and monos, with which they mix well, these fish prefer higher salinities as they mature, and the aquarist should aim to keep the salinity at or above 1.015 once they are fully grown.
Strangely, while batfish are big, predatory fish that seemingly wouldn't have much to be afraid of, they are surprisingly skittish, and will only do well with similarly placid tankmates. Apart from scats and monos, dwarf lionfish, brackish water moray eels, archerfish, Dormitator maculatus sleeper gobies, and perhaps even adult sailfin mollies would all make good companions. Also note that while the orbiculate batfish is a hardy animal, the other species of batfish are much less robust. The long-finned batfish Platax pinnatus in particular is extremely delicate, and many would say this species should be avoided by marine aquarists, let alone brackish water ones.
Fact Box: Five tips for successfully adapting marine fish in a brackish water tank
Raise the salinity in your brackish water tank to at least 1.018 before getting any of the marine species described here. The smaller the salinity difference between the dealer's tanks and your home aquarium, the easier it will be to adapt the new fish.
Marine species will only combine well with brackish water species that enjoy half to full strength seawater. This includes most of the big brackish water species such as scats, monos, archers, moray eels, and sleeper gobies. Salt-tolerant freshwater fish on the other hand, including cichlids such as kribensis and orange chromides, glassfish, guppies, spiny eels, knight and bumblebee gobies, and figure-eight puffers, cannot be kept at high salinities and cannot be mixed with marine species.
Only choose a marine fish that is obviously robust and feeding well. If the fish isn't doing well in a marine tank, things will only get worse once you put it into a brackish water system.
Remember that while these fish can adapt to reduced salinities, they are still marine fish when it comes to the need for good filtration, high oxygen levels, and large aquaria with plenty of swimming space.
Make sure you can positively identify the fish before you buy it, if in doubt, ask your dealer for the Latin name and cross check it with a marine fish identification book. Getting the right fish is crucial, in particular with the puffers and batfish, where there are similar looking species to the ones described that cannot adapt to brackish water and will probably die if you try.
Keep a close eye on your new fish, and make sure that it is behaving naturally and feeding well. Be prepared to raise the salinity or return the fish if it looks like things aren't working out.