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Oddballs: Eels, Soles & Puffers

Oddball fishes appeal to both beginners and experts alike, but it's important to note that oddballs often have very particular needs, so if you plan to keep them, you need to do your research first. In many cases, once these requirements are met, they're not difficult to maintain, but there certainly are some oddball species that can pose a challenge to even the most expert fishkeeper.

To make it easier for you to keep your oddballs successfully, we're starting a series of articles this month that will help you select the best species for your aquarium. Every other month we'll look at some of the oddball species in the trade, starting this month with three of the most popular types of oddball: spiny eels, ‘freshwater' soles, and a selection of the smaller pufferfish.

Spiny eels: approach with caution!

Perhaps the most popular oddballs, spiny eels are not easy to keep, and can't be added to the community tank without a little forethought.

To start with, spiny eels are fussy feeders; they have no interest in dried foods at all, so without question, you'll need to be using live foods like earthworms initially, and once settled, frozen foods such as bloodworms and tubifex. Since they're slow feeders as well, there's no point at all mixing them with catfish or loaches that would compete with them at dinner time.

The second big issue is the substrate you can use in the aquarium. Spiny eels are notoriously sensitive to bacterial infections of the skin, and coarse substrates like gravel seem to be especially good at getting such infections started. For the smaller Macrognathus species like the 30-cm/12-inch peacock eel Macrognathus siamensis, a fine sandy substrate such as smooth silica sand is essential.

Finally, spiny eels are predatory, so tankmates have to be chosen with care. The smaller Macrognathus species are only a threat to very small fish like neons or livebearer fry, but the bigger Mastacembelus species are much more dangerous, and should only be kept with robust tankmates of similar size.

In fact it's a very good learn the scientific name of a spiny eel on offer before you buy it. Macrognathus species tend to be smaller and do well in groups, so are generally quite easy to house with other fish; Mastacembelus species are much bigger, up to 100 cm in the case of the fire eel Mastacembelus erythrotaenia, and are often intolerant of one another. They're usually kept singly, alongside robust fish such as oscars and arowanas.

Freshwater flatfish: often nothing of the kind!

Despite often being sold as such, there are very few truly freshwater flounders or soles in the trade. The only one you're likely to see is the Asian species Brachirus harmandi, a distinctive species with mottled brown patches on its underside as well as its upper surface. Virtually all the other flounders and soles traded are either brackish water species or juveniles of a species that eventually swims into the sea once it matures.

The species most commonly seen in British shops seem to be South and Southeast Asian species of Brachirus, occasionally Brachirus harmandi, but more often Brachirus orientalis and Brachirus pan. Although the name Brachirus panoides is often mentioned by retailers, the fish sold as Brachirus panoides often turn out to be specimens of Brachirus orientalis instead.

Brachirus pan is a euryhaline species that does well at low salinities, and brackish water with a specific gravity of 1.003 to 1.010 suits it well. By contrast, Brachirus orientalis is a temporary visitor to freshwater habitats, and needs quite saline conditions in the long term; a specific gravity of 1.010 up to fully marine conditions are recommended.

With the exception of Brachirus harmandi, telling species of Brachirus apart isn't easy. They are almond-shaped when viewed from above, but there are differences in how sharply the body tapers towards the tail. Brachirus pan is about four times as long as it is broad, whereas Brachirus panoides is only about 2.5 times as long as it is broad. Brachirus orientalis is even less elongate, its length being about twice its width, and if anything, it has a more oval rather than almond-shape when viewed from above.

Soles and flounders tend to be nocturnal fish, and that needs to be considered when choosing tankmates. Don't force them to compete with bottom feeders such as loaches and catfish; instead, keep them with animals that feed slowly and during the day, such as gobies. Generally these fish have no interest in dried foods, so live and wet frozen foods are the only options. They like wormy things, so bloodworms, tubifex and even small earthworms are all on the menu. It's a sad fact that most of these fish starve to death in captivity, so take this aspect of their care seriously!

As is well known, flatfish are burrowing fish, so there's no point keeping them in a tank with gravel; they absolutely must be kept in an aquarium with smooth silica sand or, in the case of brackish water species, coral sand. Otherwise, they are completely indifferent to aquarium decoration, and ignore things like rocks and plants entirely.

Pufferfish: clowns with attitude!

With very few exceptions, pufferfish are not suitable for the community tank, so the first thing to realise when shopping for puffers is that you'll probably need to buy a whole new aquarium as well.

That said, some pufferfish are rather small, and a 45-litre/10-gallon tank makes a fine home for a group of dwarf puffers, Carinotetraodon travancoricus. A tank twice that size would work well for the rather mild-mannered red-tail puffer Carinotetraodon irrubesco, or the less frequently traded but rather aggressive red-eye puffer Carinotetraodon lorteti, a pair of which would represent a challenging but certainly do-able breeding project for the more ambitious aquarist.

All Carinotetraodon are plant-friendly fish, and look their best in tanks with plenty of vegetation. Floating plants in particular encourage them to swim to surface when hungry, while Java moss seems to be particularly favoured as a place for them to lay their eggs.

Alternatively, a 90-litre/20-gallon tank with brackish water at a specific gravity of 1.003 to 1.005 would make a nice home for two or three figure-8 puffers, Tetraodon biocellatus. This is a lovely species noted for its beautiful colouration and outgoing personality. While not a community fish by any means, it seems to get along very well with bumblebee gobies, so there's potential to mix the two species together should you want to.

The South American puffer Colomesus asellus is another very pretty species, but while no larger than the figure-8 puffer at around 8 cm/3 inches when fully grown, it's a hyperactive species that needs plenty of swimming space and a strong water current. Tanks upwards of 125-litres/30 gallons in size make sense here, allowing you to keep a group of three or four specimens without worrying too much about water quality issues. Often suggested as a good community tank species, this is only true up to a point, since it is a confirmed fin nipper. Fast-moving loaches, glassfish, barbs, and tetras can make good companions, provided the aquarium gives everyone swimming space and hiding places.

Given adequate space and proper filtration, puffers are generally hardy animals that do well across a range of water chemistry values. The main thing with puffers is to watch they aren't fed so much that water quality is compromised: these fish are expert "beggars" that will constantly spend their time mooching for more food!

How to treat oddballs with whitespot

Some aquarists have found spiny eels, soles and puffers sensitive to the copper and formalin medications used to combat whitespot, also known as ick. Unfortunately, whitespot is one of the commonest problems aquarists purchasing oddball fish have to deal with.

Fortunately, these fish have a high tolerance of salt, even those species that would not normally enter brackish water habitats in the wild. While the whitespot parasite can't be killed by salty water while embedded in the fish, but once the white cyst bursts and the free-living parasites swim about looking for new hosts, salt water quickly kills them.

To treat whitespot using salt, you need non-iodised cooking salt, sometimes called kosher salt, or plain aquarium salt rather than marine salt mix. Only small quantities are needed: 5-6 grammes per litre should do the trick nicely. Don't add the salt directly to the aquarium; instead dissolve it in a cup of warm water first.

It's a good idea to raise the temperature of the aquarium by a few degrees as well, to speed up the life cycle of the parasite; 28-30 degrees-C should be fine. Leave the water salty for at least two weeks, by which time you should find your fish entirely free of parasites. You can then do normal water changes to flush out the salt.

Other fish articles:

Other fish articles you may be interested in are listed below, click an article for full details.