Taxonomy & naming
Hemigrammus collettii was described by the Austrian ichthyologist Franz Steindachner in 1882. The authority appears in parentheses because the species was originally placed in a different genus and subsequently transferred to Hemigrammus. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) is the governing authority for the valid name.
The genus Hemigrammus Gill, 1858 is one of the largest in the family Characidae and is broadly defined by a scaled caudal fin (a feature that distinguishes it from the otherwise similar Moenkhausia, which lacks caudal-fin scales in part). Hemigrammus contains dozens of species distributed across tropical South America, and ongoing molecular work has periodically revised its limits and internal relationships. H. collettii is not currently assigned to any named species group and its precise phylogenetic placement within the genus remains unresolved at a fine scale.
The specific epithet collettii is a patronym, most likely honouring the Norwegian zoologist Robert Collett (1842–1913), though bibliographic details of the original description are not reproduced here.
Morphology
Hemigrammus collettii reaches approximately 2.5 in total length, making it a mid-sized member of a genus whose species range from tiny nano tetras to larger forms approaching 3–3.5 in. The body follows the standard Hemigrammus template: moderately compressed laterally, deepest at the dorsal-fin origin, tapering to a slender caudal peduncle. The scaled caudal fin is the genus's hallmark character.
Colouration is silvery to pale gold along the flanks, with the dark lateral stripe that is diagnostic of many small Amazonian tetras — a pigmented band running from behind the opercle to the caudal-fin base, often ending in a small caudal spot. Fins are largely hyaline (clear) in typical specimens. Colour intensity and the visibility of the lateral marking depend on light, background, and fish condition; well-settled individuals in a planted tank with dark substrate show richer contrast than stressed fish in bare holding tanks.
Sexual dimorphism is subtle, as in most Hemigrammus: ripe females are noticeably deeper-bodied and more rotund viewed from above, while males are slimmer. No conspicuous colour differences occur between the sexes.
Habitat
In the wild, Hemigrammus collettii occurs across the Amazon basin and the Guianas — a broad range spanning Peru, Brazil, Suriname, Guyana, and French Guiana. It is recorded as benthopelagic, occupying the lower to mid water column in a variety of lentic and slow lotic habitats: forest streams, flooded margins, and lagoon-like backwaters where organic matter accumulates and vegetation is dense.
Water conditions across this range vary but tend toward the soft and acidic end of the tropical spectrum. FishBase records a pH of 5.6–7.2 and hardness below 15 dH for the species, with temperatures of 73–81 °F. Blackwater and clearwater habitats both fall within this envelope. Like many Amazonian small tetras, H. collettii likely shelters among submerged root tangles, leaf litter, and low vegetation, feeding opportunistically on whatever small invertebrates the habitat provides.
Feeding
Hemigrammus collettii is an omnivore, as is typical for the genus. In nature it takes small aquatic invertebrates — insect larvae, microcrustaceans, zooplankton — along with algal filaments and organic detritus encountered during foraging in the benthopelagic zone.
In the aquarium it accepts a wide range of prepared and live or frozen foods without difficulty. A diet of good-quality micro-pellets or fine flake as a staple, supplemented with frozen or live Daphnia, brine shrimp nauplii, bloodworm, and Cyclops, maintains both health and colour. Small, frequent feedings suit the species better than single large meals, and variety appears to contribute to overall condition and willingness to spawn.
Mating
Hemigrammus collettii is an egg-scattering open spawner, following the typical characid mating pattern in which no permanent pair bond forms and no parental care is given. Males chase and display to females in the days before spawning, and spawning activity is typically triggered by a combination of conditioning on live and frozen foods, a slight drop in water temperature, and the presence of fine-leaved vegetation or spawning mops.
Captive reproduction was first documented in 1988 according to FishBase records. Spawning occurs in the early morning hours, with the male driving the female through fine vegetation where small adhesive eggs are released and fertilised in short bursts. The adults do not guard or tend the eggs and will readily consume them if not removed.
Breeding
To breed Hemigrammus collettii deliberately, condition a group or a selected pair on varied live and frozen foods for one to two weeks, then move them to a bare or lightly planted spawning tank with slightly softer, more acidic water than the holding tank — targeting pH 6.0–6.5 and temperature around 77–79 °F. Fine-leaved plants such as Java moss or synthetic spawning mops give the fish a surface over which to scatter eggs.
Eggs are small and semi-adhesive, settling among plant material after scattering. Remove the adults promptly after spawning, as they will eat the eggs. At 77–79 °F the eggs hatch in approximately 24–36 hours, and the yolk-sac fry become free-swimming within a few more days. First foods should be infusoria or commercial liquid fry foods, graduating to newly hatched brine shrimp and powdered fry flake as the young grow. Water quality must be maintained carefully at this stage, as very small fry are sensitive to ammonia and nitrite.
In the aquarium
Hemigrammus collettii is a peaceful, sociable tetra that does best in groups of eight or more individuals. Kept in smaller numbers, individuals may become shy or spend time hiding; a proper school produces the natural shoaling behaviour, with fish moving in loose formation through the mid and lower water column and showing their best colour.
Water parameters should reflect the species' natural range: soft to moderately hard water (below 15 dH), pH 5.6–7.2, and temperature 73–81 °F. It is not demanding and tolerates a wider range once established, but very hard or alkaline water should be avoided for long-term keeping and breeding. A planted aquarium with subdued lighting, a dark substrate, and some driftwood or leaf litter brings out the lateral stripe and settles the fish. Tankmates should be similarly peaceful and similarly sized — other small tetras, rasboras, dwarf corydoras, and small livebearers all work well. Larger or more aggressive fish will outcompete H. collettii at feeding and may intimidate it into permanent hiding. The species is hardy once settled and unremarkable to maintain, making it a good choice for a community planted tank.
Conservation
Hemigrammus collettii is assessed as Least Concern (LC) on the IUCN Red List, with the assessment completed on 4 March 2021. The Least Concern listing reflects the species' broad range across the Amazon basin and the Guianas, its apparent tolerance of varied habitat conditions, and the absence of evidence for significant population decline or specific threats at a range-wide scale.
The Amazon basin continues to face large-scale pressures including deforestation, agricultural runoff, and hydropower development, all of which affect small-stream characids to varying degrees. At present, however, H. collettii is not considered threatened. It is bred regularly in captivity by commercial fish farms, and most specimens in the trade are tank-raised; demand for wild-caught individuals is low relative to the population size across its extensive range.