Taxonomy & naming
Hemigrammus hyanuary was described by Mary J. Durbin in 1918, placing it in the large characid genus Hemigrammus. Eschmeyer's Catalog of Fishes records the valid name as Hemigrammus hyanuary Durbin, 1918; because the species was originally described in Hemigrammus and has remained there, no parenthetical authority is used. The type locality is Lake Hyanuary (also spelled Janauari or Janeiro), a varzea lake on the outskirts of Manaus in Amazonas state, Brazil.
Hemigrammus is a speciose genus of small characins distributed broadly across tropical South America, united by the presence of scales on the caudal fin lobes — a feature that separates the genus from the closely related Hyphessobrycon, which lacks them. H. hyanuary is a fairly typical member of the genus in body form and colouration, and its taxonomy has remained stable since the original description.
The specific epithet hyanuary refers directly to the type locality, Lake Hyanuary near Manaus; the common names January tetra and Costello tetra both circulate in the hobby, the former as a straightforward translation of the lake name and the latter of uncertain origin.
Morphology
Hemigrammus hyanuary is a slender, laterally compressed tetra reaching approximately 1.5–2 in in total length, with males typically at the smaller end and females slightly deeper-bodied when gravid. The body is silvery to straw-coloured with a faint iridescent sheen, and a narrow dark lateral stripe runs from behind the operculum to the base of the caudal fin, where it meets a small dark spot at the caudal peduncle — a pattern typical of the genus.
The fins are largely hyaline, though males in breeding condition may show faint orange or reddish tones in the caudal fin. The adipose fin, characteristic of characids, is small and unpigmented. Like other Hemigrammus, caudal fin scales are present on both lobes. Overall the colouration is understated compared with more flamboyant tetras, but the species' clean lines and active schooling behaviour make it visually appealing in a planted aquarium with good lighting.
Habitat
In the wild, H. hyanuary inhabits the calm, heavily vegetated margins of Amazon floodplain lakes, lagoons, and slow-flowing channels — the varzea and igapó ecosystems that characterise the lower Amazon and its tributaries. It favours areas with submerged root systems, overhanging bank vegetation, and accumulations of leaf litter, where structure provides cover and the water is poorly lit and tannin-stained.
Water conditions in these habitats are typically warm (73–81 °F), soft to moderately hard (GH 5–15), and slightly acidic to neutral (pH 6–7.5). The species has a broad distribution across the Amazon basin, with confirmed records from Bolivia, Brazil, Colombia, Ecuador, and Peru, suggesting tolerance of moderate variation in water chemistry across the range.
Lake Hyanuary itself, the type locality, sits in the seasonally flooded varzea belt around Manaus — a system that experiences dramatic annual water-level fluctuations of several metres, and the fish's tolerance of varied conditions likely reflects adaptation to this dynamic environment.
Feeding
Hemigrammus hyanuary is an omnivore with a tendency toward insectivory, feeding in nature on micro-crustaceans, small worms, aquatic invertebrates, and insects that fall or emerge at the water surface. The floodplain lake habitat provides a seasonally variable food supply, and the species' broad dietary tolerance reflects its opportunistic foraging strategy.
In the aquarium it accepts a wide range of prepared foods including quality flake, micro-pellets, and granules, and benefits from regular supplementation with frozen or live foods such as Daphnia, Artemia nauplii, bloodworm, and micro-worms. A varied diet maintains good condition and colouration, and helps bring fish into spawning readiness. As a small, active species it does best fed small amounts multiple times daily.
Mating
Like other members of Hemigrammus, H. hyanuary is an egg-scattering species with no parental care. Spawning is typically triggered by a combination of conditioning on live and frozen foods, slightly elevated temperature (toward the upper end of the range, around 79 °F or above), and the provision of fine-leaved plant material or spawning mops as a substrate onto which adhesive eggs are deposited.
Males actively pursue females during courtship, and spawning occurs in brief bursts as the pair or group move through the plant mass. Because there is no pair bond and no territory defended, groups of several males and females will spawn communally under the right conditions. Separating conditioned fish into a dedicated breeding tank and controlling water chemistry and temperature gives the best results for deliberate breeding attempts.
Breeding
Breeding H. hyanuary in captivity is straightforward. A dedicated breeding tank set up with soft, slightly acidic water, a thin layer of fine substrate or bare bottom, and clumps of Java moss or a synthetic spawning mop provides an ideal environment. Dimming or shading the tank encourages spawning, which typically takes place in the early morning hours.
The female scatters small, adhesive eggs among the plant material; a typical spawning may yield several dozen to over a hundred eggs. The adults take no interest in the eggs or resulting fry and will consume them if given the opportunity, so breeders remove the spawning mops or transfer the parents promptly after spawning is observed. Eggs hatch in approximately 24–48 hours at breeding temperature, and the fry become free-swimming a day or two later. First foods should be very fine — infusoria, commercial liquid fry foods, or rotifers — before progressing to newly hatched brine shrimp as the fry grow. Growth is steady under good conditions.
In the aquarium
Hemigrammus hyanuary is a peaceful, sociable tetra best maintained in groups of six or more, where it forms a cohesive school and shows natural behaviour. It is suitable for community aquariums alongside other small, non-aggressive species — other tetras, small rasboras, dwarf cichlids, Corydoras catfish, and similar fish of comparable temperament and size. It is not a fin-nipper and generally ignores tankmates.
A planted aquarium with a dark substrate, some driftwood or leaf litter, and subdued or dappled lighting replicates the tannin-stained floodplain environment and brings out the species' natural behaviour. Water should be soft to moderately hard, slightly acidic to neutral, and warm (73–81 °F). Standard aquarium filtration providing gentle water movement suits the species well; it does not thrive in strong current. The Costello tetra is an undemanding and robust fish once settled, suitable for intermediate and experienced aquarists alike, and its modest size makes it appropriate for tanks of 15 US gal or more for a proper school.
Conservation
Hemigrammus hyanuary was assessed as Least Concern (LC) on the IUCN Red List in March 2021. The assessment reflects its wide distribution across five South American countries within the Amazon basin, its occurrence across a range of habitat types, and the absence of evidence for rapid population decline.
The Amazon basin broadly faces ongoing threats from deforestation, agricultural runoff, dam construction, and climate-driven hydrological changes — pressures that affect varzea and floodplain lake ecosystems throughout the species' range. However, H. hyanuary's broad tolerance and large range mean it is not considered to be at elevated risk at present. Commercial collection for the ornamental fish trade occurs, but wild-caught individuals are common in the trade and captive-bred stock is also available; collection pressure is not believed to threaten the species given its abundance across the basin.