Taxonomy & naming
Characidium gomesi was described by the Brazilian ichthyologist Lauro Travassos in 1956. It sits in the genus Characidium within the family Crenuchidae, the South American darters — a group whose perching, bottom-oriented body plan and behaviour led early workers to compare them to Old World darters (Etheostoma) even though the two groups are not closely related; the resemblance is convergent, driven by a shared benthic, sit-and-wait lifestyle in flowing water.
Characidium is a large and taxonomically active genus of small Neotropical stream fish, and species-level identification within it relies on details of colour pattern, scale counts and fin-ray counts rather than any single obvious field mark; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the reference for the currently valid name and synonymy. Characidium gomesi is distinct from the more commonly traded C. fasciatum and other congeners, though hobby literature sometimes uses "darter characin" or "darter tetra" loosely across the genus rather than naming individual species precisely.
Morphology
Characidium gomesi is a slender, torpedo-shaped little fish reaching a maximum of about 2.5 in standard length, keeping it firmly in nano-fish territory. Like other Characidium, it has a somewhat flattened underside and a body built for holding position against current and perching on hard surfaces rather than for sustained open-water swimming, and its fins — including a well-developed adipose fin typical of characins — are proportioned for short, precise bursts rather than continuous cruising.
Colouration in the genus typically runs to a series of dark blotches or a broken lateral band over a pale to olive body, camouflage suited to resting on rock and leaf litter in flowing water; the cache consulted for this article did not carry a detailed colour-pattern description specific to C. gomesi, so keepers should consult a photographed reference for this species specifically before assuming it matches better-known congeners such as C. fasciatum.
Habitat
The species is native to the upper Paraná River drainage in Brazil, recorded from the Grande, Tietê and Paranapanema rivers — all tributary systems feeding the greater Paraná-Paraguay basin. Within these rivers, darters of this genus occupy flowing water over rock, gravel or leaf-litter substrate, where they perch rather than swim in open water, using cover and a low profile to ambush passing prey.
The upper Paraná drainage lies at a more southerly, higher-elevation latitude than the classic Amazonian blackwater systems, so conditions here run somewhat cooler and more seasonal than in equatorial South America; keepers should not assume the warm, soft blackwater profile typical of Amazon fish applies uniformly to this species without more specific water-chemistry data.
Feeding
Characidium gomesi is a micro-predator with a trophic level (recorded on FishBase at roughly 3.5) consistent with an omnivorous-to-carnivorous diet rather than a grazer or detritivore. In the wild it takes small aquatic and terrestrial insects, insect larvae and micro-crustaceans, ambushing prey from a perched position rather than actively foraging over long distances.
In the aquarium, darters in this genus are frequently reported to ignore dry flake and prepared foods, so keepers should plan on small live and frozen fare — microworm, baby brine shrimp (Artemia nauplii), Daphnia and Cyclops — as the staple diet, with frozen bloodworm accepted by related Characidium species. Because the mouth is small, food size matters as much as food type.
Mating
Specific courtship behaviour for Characidium gomesi is not documented in the sources consulted for this article. General accounts of darter characin breeding describe pairing behaviour associated with cooler water and softer, more acidic conditions that may signal a seasonal spawning trigger, echoing the upper Paraná drainage's more temperate, seasonal climate compared to equatorial river systems. Data on sexual dimorphism specific to this species is sparse; keepers attempting to breed related Characidium typically distinguish sexes by body fullness in ripe females rather than by colour or fin differences.
Breeding
Characidium as a genus are substrate egg-scatterers, not nest builders or mouthbrooders: a pair spawns over hard surfaces such as rock or fine gravel, the female releasing adhesive eggs — clutches of roughly 150 eggs are reported for darter characins in general — that receive no parental care once laid. Eggs are reported to hatch in as little as 30 to 40 hours at suitable temperatures, a fast turnaround consistent with the group's small size and simple reproductive strategy.
Spawning in related darter species has been triggered in captivity by dropping the temperature to around 79 °F alongside soft, acidic water in the pH 6.0–6.5 range — a cue that fits the species' more southerly, seasonal native range. Newly hatched fry have very small mouths and require infusoria-sized foods initially, graduating to brine shrimp nauplii as they grow; this is a breedable fish at small scale for a patient keeper, though C. gomesi-specific breeding reports were not found in the sources used here.
In the aquarium
Characidium gomesi is a genuinely nano-suitable species at a maximum size of about 2.5 in, well suited to smaller aquascaped tanks that most piranha- and pacu-scale characins cannot fit into. It is a bottom-perching fish rather than a schooler in the conventional sense, so tank design should prioritise varied hard substrate, smooth stones, driftwood and leaf litter that give it perches and sightlines to dart from, rather than open swimming space or heavy current alone.
Because it is a micro-predator that often refuses flake, keepers should be prepared to offer live or frozen foods — brine shrimp, Daphnia, microworm, bloodworm — rather than relying on prepared diets. Tankmates should be similarly small and non-aggressive; this is not a fish suited to the boisterous, food-competitive community tanks that suit larger, more opportunistic characins. Given its more temperate upper-Paraná origin, it is worth erring toward the cooler, moderate end of typical tropical fishkeeping temperatures rather than assuming Amazon-basin warmth is required.
Conservation
Characidium gomesi is assessed by the IUCN Red List as Least Concern, an assessment recorded via FishBase as dated 7 November 2018. This reflects the absence of documented major threats to the species within its upper Paraná drainage range (Grande, Tietê and Paranapanema rivers) rather than detailed population data; as with many small, non-commercial river fish, comprehensive abundance trends are not well studied.
The upper Paraná basin has been heavily modified by damming and hydropower development over the twentieth century, and river fish throughout the system face ongoing pressure from habitat alteration, water-quality change and fragmentation of free-flowing reaches. The species does not appear to be a significant part of the aquarium trade, so collection pressure specific to the hobby is likely minimal; broader basin-scale habitat change is the more relevant long-term consideration.