Taxonomy & naming
Characidium tenue was described by the American herpetologist and ichthyologist Edward Drinker Cope in 1894. It sits in the genus Characidium, part of the family Crenuchidae (the South American darters), within the order Characiformes — the same broad order that contains the tetras, piranhas and pacus covered elsewhere in this atlas, though Crenuchidae is a distinct lineage with its own body plan and bottom-dwelling habit rather than the open-water shoaling typical of most characins.
Characidium is a large genus, with roughly sixty recognised species distributed across coastal and inland South America, and species-level identification within the genus is difficult and rests on fine details of scale counts, fin-ray counts and banding pattern rather than anything visible at a glance. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the current valid combination Characidium tenue; this account keeps to that name and genus rather than attempting any reassignment.
Morphology
Characidium tenue is a small, slender, cylindrical-bodied fish; FishBase records a maximum length of 2 in, placing it toward the smaller end of the genus (other Characidium species reported in the trade literature run to roughly 3 in). Like other darters, it has a reduced swim bladder — an adaptation that keeps it negatively buoyant and comfortable resting on the bottom rather than holding position in the water column — and paired fins positioned to let it prop itself against the substrate.
Colouration in Characidium species generally follows a cryptic, substrate-matching pattern of dusky blotches or a broken lateral band over a pale to olive body, which camouflages the fish against sand and gravel between darting movements. No colour or pattern description specific to C. tenue was found in the cache and none is asserted here; general genus appearance is described only to orient the reader.
Habitat
Characidium tenue is native to temperate freshwater systems of southern South America — FishBase documents it from the Uruguay River and Lagoa dos Patos basins in Brazil, with additional reports from Argentina. This is a notably more temperate range than the classic Amazonian blackwater associated with many characins, and it matters for care: this is a subtropical species from a basin that runs cooler than the equatorial Amazon.
Within that range FishBase describes it occupying a wide variety of habitats, from large sandy beaches in open lagoon systems to smaller muddy or rocky streams with slow current, and swampy environments — a generalist darter rather than a narrow rapids specialist. Other Characidium species in the hobby literature are associated with fast-flowing, highly oxygenated water near rapids and waterfalls, so a sandy or fine-gravel substrate with reasonable water movement and strong oxygenation is a sensible match for the genus even where C. tenue itself is reported from calmer habitats.
Feeding
FishBase assigns Characidium tenue a trophic level of 3.4, consistent with a micro-predator rather than an omnivore or grazer. Genus-level accounts describe Characidium darters feeding primarily on aquatic and terrestrial insects and chironomid (midge) larvae, taken in short darting strikes from a perched resting position rather than pursued through open water.
In the aquarium this translates to a fish that often ignores flake and should not be expected to compete for food at the surface. Frozen or live bloodworm, brine shrimp, Daphnia and Cyclops are readily taken, and some individuals will accept sinking micropellets or tablets once settled, but these should supplement rather than replace small live or frozen foods. Feeding needs to reach the bottom, where the fish actually forages.
Mating
No species-specific courtship account for Characidium tenue was found in the cache. Crenuchid darters in this genus are described in hobby literature as gregarious fish best kept in a real group, and conditioning ahead of any spawning attempt would follow the general genus pattern — soft, acidic water, good movement and fine-leaved plant cover to trigger and support spawning behaviour.
External sexual dimorphism is not documented for this species in the cache and is not asserted here.
Breeding
Characidium darters, including reports for the genus more broadly, are egg-scatterers over fine-leaved plants and sediment rather than nest-builders or mouthbrooders, and parental care is minimal to absent — this contrasts with the true nest-guarding of the piranhas and the brief male cave-care seen in the sailfin crenuchids (Crenuchus, Poecilocharax) elsewhere in this family. Hobby accounts for the genus describe conditioning a pair or small group in a separate tank with soft, acidic water, fine-leaved plants such as Java moss, and good water movement; several hundred eggs may fall between rocks or plant strands and hatch in roughly 30–40 hours at warm temperatures, with no further parental involvement.
No breeding record specific to Characidium tenue was found in the cache, so the above should be read as general Characidium biology rather than a confirmed account for this species; fry, once free-swimming, would be expected to need infusoria followed by baby brine shrimp and finely crushed foods, consistent with other small characins.
In the aquarium
Characidium tenue stays small — around 2–2.5 in — but it is a bottom-perching specialist rather than a typical mid-water tetra, and its care should reflect that. A fine sand or gravel substrate, moderate-to-strong water movement, and strong oxygenation suit the genus well, and a tight-fitting cover is essential: Characidium darters are noted as expert jumpers. Genus accounts recommend keeping the fish in a group of at least five, and some sources suggest considerably more, since a solitary or thinly-stocked darter is a more nervous, less visible fish.
Given the temperate, cooler-running Uruguay River and Lagoa dos Patos basins this species comes from, water on the cooler side of typical tropical community-tank temperatures is appropriate rather than a high-end tropical setup; pH in the slightly acidic-to-neutral range and clean, well-filtered water with low nitrate are consistently emphasised across the hobby sources, since darters are reported as intolerant of poor water quality. Feed to the bottom with small live or frozen foods rather than relying on flake, and expect a subtle, substrate-hugging fish rather than an active mid-water swimmer — its appeal is close observation of natural darting, perching behaviour, not colour or activity in open water.
Conservation
Characidium tenue has not been evaluated by the IUCN Red List; no assessment for the species was found. This is common among smaller, regionally restricted South American characins that fall outside the largest commercial fisheries or aquarium-trade volumes, and a Not Evaluated status should be read as a lack of assessment rather than evidence the species is secure or in decline.
No cache source documented specific conservation pressures on this species. General pressures facing temperate South American river systems — water quality, damming and land-use change in the Uruguay River and Lagoa dos Patos drainages — are plausible long-term watch items for a range-restricted species like this one, but no source-specific claim is made here beyond that general context.