Taxonomy & naming
Nannocharax rubensteini was described by Jerep and Vari in 2013 in the journal Copeia, originally placed in the genus Hemigrammocharax. It belongs to the family Distichodontidae — the African characins that also include the plant-grazing Distichodus and the deep-bodied citharinid food fishes — within the order Characiformes. The original description highlighted a distinctive coloration and the first reported presence of a pseudotympanum (a small membrane-covered structure near the swim bladder, used in sound reception or production) within Distichodontidae, a notable morphological detail for the family.
Around 2014, a subsequent Copeia paper on new Nannocharax species placed Hemigrammocharax into synonymy with Nannocharax, making the genus monophyletic; the species is accordingly now recognized as Nannocharax rubensteini rather than under its original combination. Nannocharax is a genus of small to very small African river characins — FishBase lists more than 25 species, with adult sizes as modest as 0.5 in SL in N. brevis and around 2 in SL in some of the larger congeners — and the genus name itself derives from the Latin/Greek "nannus," meaning small or dwarf.
Morphology
Detailed morphometric data for Nannocharax rubensteini is sparse in publicly accessible sources; the original description's morphometric table was not retrievable for this account. What is documented is that the type specimen measured approximately 1 in (about 1 in) standard length, making it one of the smaller described species in an already diminutive genus. Based on its close relatives, it can be expected to share the general Nannocharax body plan: an elongated, slender body, a small terminal or subterminal mouth suited to picking at small prey and biofilm, and the family's characteristic small adipose fin.
The original description notes a distinctive coloration for the species (used, alongside the pseudotympanum, to separate it from congeners), but a specific pattern description was not available in the cached sources. True adult maximum size is not firmly established beyond the type specimen; given the size range of related Nannocharax species, an adult maximum in the region of 1 in SL is a reasonable expectation, but this is an estimate, not a documented figure.
Habitat
Nannocharax rubensteini is known from the Lékoli River, part of the Likouala River system in the northwestern Congo River basin, Republic of the Congo — a single, narrow range typical of many small Distichodontidae. The Likouala system runs through dense equatorial swamp forest with strong seasonal flooding, and its waters are characteristically acidic and heavily tannin-stained, with humic content that colors the water reddish-brown.
Water chemistry in this part of the Congo basin is markedly soft and acidic — typical pH in the 3.5–5.2 range — with the wider Congo basin running roughly 68–81 °F, though individual tributaries and swamp-forest streams can run cooler. This is a genuinely extreme, blackwater-adjacent environment, and it should not be conflated with either the moderate West African rivers some other Distichodontidae inhabit or with South American blackwater parameters, which differ in mineral content and buffering.
Feeding
Specific dietary data for Nannocharax rubensteini has not been documented in the sources available for this account. Based on the feeding ecology described for the genus, Nannocharax species are generally carnivorous benthivores, taking small invertebrates and likely some epiphytic material from stream-bottom substrates, rather than being plant-grazers in the manner of their larger distichodontid relatives such as Distichodus.
In husbandry accounts of related Nannocharax species, live and frozen foods such as bloodworm and Daphnia are strongly preferred, with dry foods rarely accepted — consistent with a genus adapted to picking live prey from current-swept substrate rather than grazing plant material or scavenging prepared food.
Mating
No courtship or spawning behavior has been documented for Nannocharax rubensteini specifically. Egg-scattering Distichodontidae in general — including the better-known Distichodus and citharinids — show little in the way of described pair-bonding or courtship display, and nothing more specific can be responsibly stated for this species without direct observation. Data sparse.
Breeding
Breeding has not been documented in the aquarium or in the wild for Nannocharax rubensteini. Distichodontidae as a family are, with few exceptions, egg-scatterers with no parental care, and that mode is presumed to apply here by analogy with the family, but it is unconfirmed for this particular species. Given how rarely any Nannocharax reaches the aquarium trade, and that this species has no aquarium-keeping history at all, there is essentially no prospect of captive breeding being attempted in the near term. This account reports that honestly rather than inventing spawning details that have never been recorded.
In the aquarium
Nannocharax rubensteini has no established presence in the aquarium hobby — it is not traded, and no husbandry reports specific to the species exist. Extrapolating cautiously from care information for the more available congener N. brevis, small Nannocharax species are kept in modest tanks (around 10 US gal), at 73–81 °F and pH 6.0–7.5, with soft water, strong but gentle current, a well-oxygenated setup, and live or frozen foods essential rather than optional, since dry foods are usually refused. They are described as sensitive to organic waste buildup and needing regular partial water changes.
Given the far more extreme, acidic water (pH 3.5–5.2) of its actual home river and its tiny recorded size (roughly 1 in SL), any prospective keeper — should this species ever appear in trade, which is unlikely — should expect a highly specialized nano fish requiring soft, acidic, well-oxygenated, current-swept water in a small group setup, not a generalist community fish. At present this is a species known essentially only from scientific collection, not from the hobby.
Conservation
Nannocharax rubensteini has not been assessed by the IUCN Red List; no formal conservation status is currently available, so it is treated here as Not Evaluated. It is known only from its type locality in the Lékoli River (Likouala system, Republic of the Congo), and no broader distribution or population data were found in the sources consulted for this account.
As with many recently described, single-river African characins, the lack of an assessment reflects limited survey effort rather than any known population trend. Congo basin swamp-forest streams face general regional pressures — deforestation, artisanal mining and localized water-quality change — but no species-specific threat data exists for this fish, and none should be assumed. Data sparse.