Taxonomy & naming
Chalceus labrosus was described by the Scottish naturalist William Jardine and the explorer Robert Schomburgk in 1841, based on material from the Guiana region. It has not held up as a valid name: current taxonomic treatment (reflected in Catalog of Fishes-derived synonymy databases such as Wikidata's species-concept records) places it as a junior synonym of Brycon falcatus Müller & Troschel, 1844, and specifically as a nomen oblitum — a name that, despite technically having priority by a few years, has been so rarely used in the literature since that the later, widely-adopted name (Brycon falcatus) is retained by convention rather than displaced.
Brycon falcatus itself sits in the family Bryconidae, one of several lineages that used to be lumped into a broader "characin" grab-bag alongside the true piranhas and pacus of Serrasalmidae before molecular and morphological work split the group. It is not closely related to Chalceus proper (the genus of true "pink-tailed" and "yellow-tailed" chalceus popular in the trade); the shared genus name here is an artefact of nineteenth-century naming rather than a current classification. Anglers and the aquarium trade know the fish as the blacktail brycon, a common name that refers correctly to Brycon falcatus and is used throughout the rest of this article for clarity, while the taxon record for the historical name Chalceus labrosus is kept under this slug for completeness.
Morphology
The blacktail brycon is a substantial, laterally compressed characin with a deep, muscular body built for sustained swimming rather than the sharp acceleration of a piranha. FishBase records a maximum length of 19.5 in standard length in females and 19.5 in SL in males — close to half a metre — making this one of the larger "minor characins" in the Neotropical fauna, well beyond typical community-tank scale.
Colouration follows the brycon pattern: bright silver flanks with a coppery or golden sheen, and the dark, often black-edged caudal fin that gives the species its common name. The mouth is set for an omnivorous, opportunistic diet — capable of taking both plant material and animal prey — and the body proportions and fin placement are consistent with an active, open-water swimmer that also patrols the surface layer, a trait tied directly to its feeding ecology (see Feeding, below).
Habitat
The species is native to South America, recorded from the Amazon basin, the Orinoco basin and the rivers of the Guyana shield, with its type locality in Guyana and Suriname. It occupies benthopelagic habitat — moving through the water column rather than hugging the bottom or the surface exclusively — in river channels and associated floodplain habitat typical of the region.
FishBase gives a water envelope of roughly 64–77 °F and pH 5.5–7.5, a broader and somewhat cooler range than many purely equatorial Amazonian characins, consistent with a wide-ranging species that crosses several river systems and elevations. As with many South American river fish, seasonal flooding is central to its ecology: reports tie spawning activity to the rainy season, when rivers rise into marginal forest and floodplain (see Mating and Breeding).
Feeding
Brycon falcatus is a genuine omnivore. In the wild it feeds heavily on fruits and seeds that fall or are shaken from trees overhanging the water, a foraging habit shared with several other Bryconidae, and it is documented jumping clear of the surface to pluck fruit and seeds directly from low branches — a striking, athletic feeding behaviour rather than an incidental one. Alongside plant material, it takes aquatic invertebrates and small fish, rounding out a diet that spans several trophic levels.
In the aquarium, keepers report the species accepting live fish, crustaceans, chopped meat and pellet foods readily; its size and activity level mean it needs a substantial, varied ration rather than the flake-and-pellet regime that suffices for small tetras. Given its wild reliance on fruit and seed material, offering some plant-based content alongside protein is a reasonable extension of the documented diet, though this specific practice is not confirmed in the sources at hand.
Mating
Little detail on courtship behaviour specifically is documented in the available sources. What is recorded is that spawning activity in the wild correlates with the onset of the rainy season (reported as roughly October to December across parts of its range), the same seasonal flood pulse that triggers reproduction in many other Amazonian and Orinocan characins. No reliable external sexual dimorphism is documented for this species in the sources reviewed; data are sparse enough here that this account does not speculate beyond what is recorded.
Breeding
Breeding is rarely achieved in captivity and is explicitly documented as difficult: aquarium and aquaculture accounts note that successful spawning has occurred only in very large tanks, and it is not a routine hobby or commercial breeding proposition the way many smaller tetras and even some piranhas are. This lines up with its ecology as a large, migratory-leaning river fish whose reproduction is tied to seasonal flood cues that are difficult to replicate outside large aquaculture facilities.
Given the scarcity of documented breeding detail, this account does not assign a specific spawning mode (egg-scattering, nest-building or otherwise) beyond noting that captive breeding is rare and confined to unusually large systems. Prospective breeders should treat any successful spawning as a notable event rather than an expected outcome of routine conditioning.
In the aquarium
The single most important fact for a keeper is size: this is not a small characin. At up to roughly 19.5 in SL, the blacktail brycon needs a genuinely large aquarium — think several hundred litres at minimum for an adult — with strong filtration and open swimming space, since it is an active, powerful mid-water cruiser rather than a fish content to hover in a corner. Small or standard community tanks are not appropriate for a fish approaching half a metre in length.
Because of its size and activity, hobby accounts flag real aggression potential when the fish is kept cramped; adequate space reduces this risk substantially. Its documented habit of leaping to take food from above the waterline is also worth planning for — a secure, well-fitted lid is a practical necessity, not an optional precaution, for a fish with this behaviour. Diet should be varied (see Feeding) and tankmates should be robust, similarly large fish able to hold their own in an active, mixed-diet setup; this is a specialist's fish, suited to large, well-established systems rather than typical home aquaria.
Conservation
Brycon falcatus is assessed by the IUCN Red List as Least Concern, an assessment dated 2020, reflecting a wide distribution across the Amazon, Orinoco and Guyana shield river systems and no documented major population decline. It is not a species of particular conservation concern based on the available assessment.
As with many wide-ranging South American river characins, the longer-term watch items are the general pressures facing Amazonian, Orinocan and Guiana shield freshwater systems — damming, deforestation-driven sedimentation and water-quality change — rather than any species-specific threat identified in the current record.