They may seem the waifs and strays of the catfish community, but the dwarf Corydoras have a charm all of their own. Nathan Hill explains more.
Some catfish keepers pine for saddle-snouted Scleromystax that snuffle across the substrate and unceremoniously sideswipe each other. Others prefer bulging Brochis that sit there winking between slapdash bursts to the surface to gulp air.
However, a little deeper in the Callichthyidae world are more subtle prizes. They are the small fish — the ones usually hidden away in dealers’ tanks.
These lack intense colours or the brazen behaviour of their chunkier kin, yet have special appeal. They are the dwarf Corydoras — and of the true dwarf cats three are readily available: C. pygmaeus, C. hastatus and C. habrosus (pictured above).
There are more and C. cochui, C. multimaculatus and C. piragua are equally diminutive, but these are so infrequently seen in the trade that it’s scarcely worth me encouraging real world aquarists to go and seek them out.
There are also the eternally cute Aspidoras cats which differ from Corydoras in head and fin structure. These are becoming increasingly familiar and several species are frequently imported. Although not inaccessible, they tend to command a steeper price than their cousins and shoals may be unrealistic for the average hobbyist.
Yet there’s more than financial consideration when opting for dwarf Corydoras. There’s also the way in which these cats differ from their kin.
True to their feline namesakes, many corys are happy to bed down into life domestic, nestling and doing nothing, awaiting their next meal and gradually getting podgy.
However, dwarf cats tend to steer clear of this hedonistic path, clinging to their shoaling roots and behaving as though their enclosed home was no different from their Amazonian habitats.
Then there’s the interaction. C. hastatus, in particular (pictured above) is well noted for its commensalism involving species of similarly marked tetra. Indeed, finding the right species of tetra for a clutch of hastatus to shoal alongside has become something of a holy grail for those in the know.
How do I keep them?
As with so many South American fish, the ideal dwarf cory tank is often thought to be riverine, with fast flowing water and a handful of straggling plants and assorted rocks and wood.
Reality, however, involves slow or no moving water, with few rocks, but much in the way of soft, dark substrates, fallen wood, and dense planting.
C. habrosus, for example, is often found in periodic wetlands, large floodplains to where other fish seasonally migrate. However, it remains behind, suffering from spats of dying off during dry seasons and abundance during the wet. With so much cover, the fish can roam moderately freely, providing that they don’t venture too far. A typical habrosus habitat is shown above (picture by Hans-Georg Evers).
Neighbours for wild habrosus are an unusual mix, many of which we may not expect to find together. Discarding what we usually think of as a South American community tank, C. habrosus are found alongside fish as diverse as guppies, Characidium, Leporinus and even Rachovia maculipinnis killifish.
Tetras also live with them and other Corydoras, like C. aeneus and C. septentrionalis, are part of the biotope. Wild predators for those foolish enough to venture into the open include Hoplias wolf fish and Pygocentrus caribe piranha.
For all three of these species, suitable tank mates would be those found in a typical slow moving river or pool in South America. Think shallow-bodied tetras, peaceful suckermouths like Otocinclus affinis, other Corydoras and surface-dwelling hatchetfish.
Avoid any fish that could fit a dwarf Corydoras into its mouth as, given the catfishes’ defensive locking spines, such a mistake would spell disaster for both predator and prey— resulting in an easily avoidable snackrifice.
Planting should be heavy, if not intense, with a mix of broad leafed and grassy greenery in equal measure. Although this may require extra lighting to successfully propagate, the pygmies will prefer this to an open, dimmed aquarium where they may feel exposed and threatened.
Shoreline inhabitors, like hastatus and pygmaeus, are often found in regions with quantities of cyperaceans (sedges) and grasses in abundance.
As with all Corydoras, pay attention to choice of substrate — going for softer, sand-themed bases. Avoid sharp sands and don’t be afraid to add a generous layer of leaf litter. The pygmies won’t spend too much time among it, but seem content from knowing that it’s there.
These fish tolerate temperatures between 22-26°C/72-79°F with 24 or 25°C/75-77°F) optimal. Soft, acidic water should be provided, with a pH value of between 6.0 and 7.2 pH, and a hardness below 15°DH.
Corydoras tetra: Who's mimicking who?
The role of C. hastatus in emulating the markings and swimming behaviour of certain, similarly marked tetras in the wild interests many enthusiasts.
Shoaling is advantageous to fishes, especially from a predation perspective, and the motto is usually ‘the more the merrier.’ As well as shoaling among their own kind, C. hastatus will often school alongside one of a few varieties of tetras — and these are sometimes imported alongside each other as by-catch.
Several species share the markings of C. hastatus, but some are better suited to co-habit than others.
The favourite contender for a tank-suitable commensal is Hyphessobrycon elachys (above) and many keepers report success with these two species together.
Odontostilbe kreigi is a similarly marked fish, but has a short temper. I have found it turning on the catfish after a few days, maybe shifting their usually insular and gregarious behaviour towards these new and unwitting shoal members.
Size of tank was not a factor, as I saw this hostility both in a 60 x 30 x 30cm/24 x12 x 12” aquarium, as well as a 100 x 100 x 100cm/40 x 40 x 40” cube.
Other possibles are the rarely seen Gnathocharax steindachneri, the more common Odontostilbe pulchra and Aphyocharax paraguayensis (above) – although the latter may be no better than kreigi in curtailing their hostile tendencies.
Three of the best
My favoured dwarf cats — pygmaeus (pictured above) hastatus and habrosus — are dwarfs by any standard. The largest barely reaches 3.5cm/1.4” and all could easily hide in the shadows of tetras.
Their size lends them to smaller aquaria, but don’t go to extremes with how tiny a home you can get away with. All too often they are recommended for the dinkiest of nano aquaria, but this can be detrimental to their shoaling and roaming behaviour. It also restricts the quantities that can be kept and, being obligate shoalers, they need lots of company.
Popular literature cites numbers of five or more as constituting a shoal, but, in many cases, this is woefully inadequate and shows our total lack of understanding about what a shoal actually is.
I wouldn’t consider anything under 12 of any one species, although in practice I’d be inclined to keep these fish in groups of twenties or thirties. Even this would not rival the teeming herds of catfish as seen in the wild.
In the larger aquarium these fish will flock nicely together, although C. habrosus less so than the others. Unusually for a Corydoras, these fish spend much of their time in midwater rather than sat on the base of the tank. In fact, especially in the case of C. pygmaeus, broad leaves of plants, well away from the substrate, would seem the preferred resting spot.
Peak activity for these species is during the twilight hours of dawn and dusk when they actually start to lose some shoaling behaviour — in contrast to other Corydoras which enhance theirs at this time — and move higher into the water column to feed.
A keeper who uses light well to exploit this activity will see the best of these fish.
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