Taxonomy & naming
Characidium alipioi was described by the Brazilian ichthyologist Edgard Alexandre Vieira Travassos in 1955, in the Arquivos do Museu Nacional do Rio de Janeiro (volume 42, pages 613–618). It belongs to the family Crenuchidae — the South American darters and their relatives, including the sailfin characins Crenuchus and Poecilocharax — within the order Characiformes. Crenuchidae was for a long time treated as a subfamily of Characidae, but is now widely recognised as its own family; Characidium is by far its largest genus, with well over 60 described species spread across Amazonian, coastal Brazilian and Río de la Plata drainages.
The species epithet honours Alípio de Miranda-Ribeiro (1874–1939), the influential Brazilian ichthyologist and museum naturalist who described a large number of Neotropical fishes in the early twentieth century — a fitting dedication given Travassos's own career at Brazil's national museum. Characidium alipioi is one of a number of Characidium species that are effectively short-range endemics, restricted to a single river basin, which makes basin-level habitat pressures directly relevant to the species' status (see "Conservation").
Morphology
Characidium alipioi is a small, elongate, sub-cylindrical characin reaching a maximum recorded length of about 2.5 in standard length, placing it firmly among the nano-sized aquarium fishes. Like other members of the genus, it has a torpedo-shaped body built for short, controlled bursts of movement along the substrate rather than sustained mid-water swimming, and its common name — flatfin darter — points to the characteristically low, often barred or spotted fins typical of the genus.
Detailed published descriptions of colour pattern and fin-ray counts for this specific species are sparse in the sources available here; Characidium as a genus generally shows a dark lateral stripe or a series of blotches along the flank, a pale belly, and subtle barring that provides camouflage against gravel and leaf-litter substrate — consistent with its life as a bottom-perching, cryptic micro-predator rather than a boldly patterned open-water fish.
Habitat
The species is endemic to the Paraíba do Sul River basin in southeastern Brazil, a river system that runs largely through the states of São Paulo, Minas Gerais and Rio de Janeiro before reaching the Atlantic — placing this darter in a subtropical rather than equatorial part of South America. FishBase classifies it as benthopelagic and tropical in habit, living on or near the stream bed of flowing freshwater.
As a stream-bed darter, C. alipioi is associated with the same kind of habitat structure that other Characidium favour: clear to lightly stained flowing water over gravel, sand or fine sediment, with leaf litter and submerged root tangles offering cover and foraging ground. Because the Paraíba do Sul basin lies at higher latitude and elevation than the equatorial Amazon, water temperatures there run cooler on average than the classic blackwater rivers further north — a point worth remembering when setting up a tank for this species rather than defaulting to warm, deep-blackwater assumptions.
Feeding
Characidium alipioi is a micro-predator. FishBase records a trophic level around the mid-to-upper end of the invertebrate-feeding range, consistent with a diet of small aquatic invertebrates rather than detritus or plant matter. Hobby accounts of Characidium in general describe a diet dominated by chironomid (midge) larvae and other small fly larvae picked from the substrate, with frozen bloodworm reported as a readily accepted substitute in the aquarium.
These darters are not reliable flake-feeders; a fish that spends its life perching and darting after tiny prey on the stream bed often ignores food floating or drifting in open water. Small live and frozen foods — microworm, baby brine shrimp, cyclops, chopped bloodworm — are the dependable way to condition and maintain this species, and food should be offered where the fish actually forages, near the substrate.
Mating
No species-specific courtship description for Characidium alipioi is available in the sources consulted here. For the genus more broadly, and for its close relatives in Crenuchidae, breeding activity is generally associated with the fine-leaved plant cover and open, well-oxygenated water typical of clear headwater and tributary streams; hobbyists breeding Characidium report that soft, acidic water and good circulation encourage spawning behaviour. Sexual dimorphism has not been documented for this species in the material reviewed; data here is sparse and should not be extrapolated beyond what is stated.
Breeding
Characidium darters are egg-scatterers: rather than building a nest or guarding a brood site the way their cave-spawning Crenuchidae relatives Crenuchus and Poecilocharax do, Characidium species scatter adhesive eggs over fine-leaved plants or fine substrate, with no parental care of eggs or fry reported. Hobby guidance for the genus recommends soft, acidic water, dense fine-leaved planting such as Java moss as an egg trap, and strong but well-directed water movement with a protected filter intake to mimic the flowing water the fish spawns in naturally.
Captive breeding reports specific to Characidium alipioi are not available in the sources used here; this account follows the documented pattern for the genus rather than a confirmed record for this species, and that distinction should be kept in mind by anyone attempting to breed it.
In the aquarium
At a maximum of roughly 2.5 in, Characidium alipioi is a genuine nano fish, but it is a bottom-dwelling stream specialist rather than a typical shoaling nano tetra, and its tank should be built around that behaviour: a longer-footprint aquarium with a sand or fine-gravel base, driftwood and leaf litter for cover, and good water movement and oxygenation rather than still water. General Characidium care guidance recommends keeping the fish either singly or in a proper group of eight or more — small numbers of a shoaling-adjacent bottom fish can result in bullying, while a real group spreads out and settles across the substrate.
Water should be kept soft and mildly acidic to neutral — guidance for the genus cites roughly pH 6.5–7.0, very soft — and, given this species' subtropical Paraíba do Sul origin, it likely does not need or want the warm, deep-tropical temperatures suited to equatorial Amazon fish; a moderate, stable temperature is a safer default than pushing toward the upper 20s °C. As a micro-predator that often ignores flake, expect to feed small live or frozen foods rather than relying on staple flake or pellet alone. This species is rarely seen in the aquarium trade, and specific husbandry data beyond general Characidium care is sparse — approach it as an under-documented, specialist import rather than an established staple.
Conservation
The IUCN Red List assesses Characidium alipioi as Least Concern, with the assessment dated 7 November 2018. As a single-basin endemic restricted to the Paraíba do Sul River system — one of the more heavily developed and industrialised river basins in Brazil, running through the densely populated states of São Paulo, Minas Gerais and Rio de Janeiro — this Least Concern status should be read alongside that context rather than assumed to mean the species faces no pressure at all; the sources reviewed here do not detail population trend or specific threats beyond the LC listing itself.
The species is not known to be a significant part of the aquarium trade, and no wild-collection trade data is available in the material used for this article; data here is sparse, and any claims about trade volume or collection pressure beyond that would be speculation.