Taxonomy & naming
Microcharacidium gnomus was described by Paulo Buckup in 1993 as part of a broader revision of the subfamily Characidiinae (Teleostei: Characiformes), a paper that erected four new genera — including Microcharacidium — and ten new species of darter characins. It sits in the family Crenuchidae, the South American darters and sailfin characins, a group whose members are unified less by size than by their perching, bottom-oriented habit in flowing water.
The genus name signals its defining trait plainly: this is a scaled-down Characidium, the older and more familiar darter genus, distinguished by a suite of technical characters from Buckup's revision (fin-ray counts, squamation and osteology) rather than by anything visible to a casual observer. The species epithet, gnomus, is the standard scientific-Latin rendering of "gnome" — a nod to the fish's diminutive stature, in keeping with a genus built around miniaturisation within an already small-bodied family.
Morphology
Microcharacidium gnomus is one of the smallest characiform fishes known, with a maximum recorded standard length of just 1 in — smaller than many aquarium shrimp. Like other crenuchid darters it has an elongate, slightly compressed body built for perching rather than sustained cruising: the paired fins are used to prop the fish against the substrate between short, quick darts, the behaviour that gives the whole family its common name.
Detailed colour pattern and fin-ray counts are part of Buckup's original technical description rather than commonly reproduced in hobby literature, and no aquarium photography or trade documentation was found in the cache for this species; keepers should treat any online image labelled M. gnomus with caution unless it traces to a verifiable source.
Habitat
The species is known from the Rio Negro basin above Barcelos in Brazil and the Orinoco River basin above La Urbana in Venezuela — two separated Amazon/Orinoco headwater-and-upper-basin systems, consistent with the localised, patchy distributions typical of small-bodied Characidiinae. FishBase classifies it as a freshwater, pelagic, tropical species, though for a fish of this size and habit "pelagic" reflects water-column classification conventions rather than an open-water lifestyle — darters of this family are characteristically substrate-associated in flowing water.
Detailed water chemistry for the type localities was not captured in the cache. Related darter characins are documented as needing strong current and high dissolved oxygen, consistent with riffle and flowing-water habitats rather than still backwaters; treat specific pH and hardness figures for M. gnomus itself as data sparse pending a dedicated source.
Feeding
FishBase records a trophic level of 3.1 for this species, placing it as a carnivorous micro-predator rather than an omnivore or grazer — consistent with the broader crenuchid darter pattern of picking off tiny invertebrates, insect larvae and micro-crustaceans from the substrate and drifting current rather than taking plant matter or detritus.
No aquarium feeding reports specific to M. gnomus were found in the cache. Given its 1 in adult size and micro-predator biology, it would need correspondingly small live or frozen foods — microworm, baby brine shrimp, cyclops and similar — and, as with many small darters, is likely to ignore flake and larger dry foods entirely.
Mating
No courtship or pairing behaviour specific to Microcharacidium gnomus was found in the cache. Within Crenuchidae, the sailfin characins Crenuchus and Poecilocharax are documented cave- and leaf-spawners with brief male attendance at the clutch, while stream-bed Characidium species scatter eggs over the substrate without forming a nest; where M. gnomus falls on that spectrum has not been documented in the sources available here, and this article does not assume one mode over the other.
Breeding
There is no documented aquarium or field breeding account for Microcharacidium gnomus in the cache, and no evidence it has been bred in the hobby. Practical Fishkeeping's coverage of related darter characins describes them as hardy and straightforward to breed given proper conditioning and strong water movement, which suggests the genus is not inherently difficult — but that account does not name this species specifically, so it should be read as a family-level data point, not a confirmed report for M. gnomus.
In the aquarium
At a maximum of 1 in, Microcharacidium gnomus would be one of the smallest true nano fish available if it ever entered the hobby in any numbers — no trade or availability data was found in the cache, and it should be treated as essentially unavailable rather than assumed to be a realistic acquisition target. Related darter characins covered in the cache are described as needing oversized filtration (roughly 10–20x tank volume) to replicate the strong current and high oxygenation of their flowing-water habitat, with a minimum tank size around 23.5 in/24 inches cited for those larger relatives; a species this small would in principle suit an even smaller footprint, provided current and oxygenation needs are still met.
As a micro-predator, any aquarium diet would need to consist of appropriately sized live or frozen foods rather than flake. Given the total absence of hobby-specific husbandry data for this exact species, any care regime should be extrapolated cautiously from its darter relatives rather than treated as established.
Conservation
The IUCN Red List assessed Microcharacidium gnomus as Least Concern in June 2020, per FishBase's citation of that assessment; FishBase also notes low vulnerability to fishing pressure and no threat to humans. This LC status likely reflects the absence of known major threats across its Rio Negro and upper Orinoco range rather than a well-studied, secure population — as is common for small-bodied, poorly documented characins with restricted, patchy distributions.
No aquarium trade pressure, habitat loss, or other specific threats were identified in the cache for this species. Its native range sits within the broader Amazon and Orinoco basins, where deforestation-linked sedimentation and water-quality change are long-term watch items for freshwater fish generally, even though nothing in the available sources ties those pressures specifically to M. gnomus.