Taxonomy & naming
Hemigrammus coeruleus was described by Mary Durbin in 1908. Eschmeyer's Catalog of Fishes (CAS) recognises the valid name as Hemigrammus coeruleus Durbin, 1908; the authority is unparenthesised because the species was described in the genus where it still resides. The genus Hemigrammus is one of the most speciose characid genera in South America, and the limits and relationships among its members continue to be revised using morphological and molecular evidence.
The species belongs to the family Characidae in the broad sense used by the Catalog of Fishes. Within the hobby it circulates under the trade names red laser tetra and blue tetra, names that reflect the iridescent lateral stripe rather than any agreed taxonomic grouping.
Morphology
Hemigrammus coeruleus is a slender, laterally compressed tetra reaching a maximum recorded total length of about 2.5 in for males and unsexed individuals. The body plan is typical of the genus: a fusiform shape with a moderately large eye, a small, terminal mouth, a forked caudal fin, and a small adipose fin between the dorsal and the caudal.
The most distinctive feature is a bright, iridescent horizontal stripe — often appearing blue or silver under aquarium lighting — that runs along the midlateral line from behind the operculum to the base of the caudal fin. This reflective stripe gives rise to the hobby name red laser tetra when the fish displays reddish caudal or body coloration under certain lighting. Colouration intensity varies with condition, lighting angle, and social state.
Habitat
The species is native to South America, recorded from the Solimões River basin and the lower Negro River basin in Brazil. Both the Solimões (the Brazilian name for the upper Amazon's main channel) and the Negro are major Amazonian drainages with strikingly different water chemistries: the Solimões carries warm, whitewater with higher conductivity, while the Negro is a blackwater river with very low pH and ion-poor, tea-stained water. FishBase records H. coeruleus as a benthopelagic, freshwater fish of tropical waters.
Natural habitat details are sparse in the available literature. As with many small Amazonian tetras, the species likely inhabits the margins of rivers and flooded forests (igapó and várzea), where overhanging or submerged vegetation provides shelter and insect prey falls to the water surface.
Feeding
FishBase classifies Hemigrammus coeruleus as insectivorous in the wild, consistent with the feeding biology of most small Hemigrammus. In their natural habitat these tetras take small invertebrates — insect larvae, microcrustaceans, and terrestrial insects at the water surface.
In aquaria the species is an omnivore that readily accepts a wide variety of prepared and live foods. Aquarists report good results with high-quality flake and small micropellets as a staple, supplemented with frozen and live foods such as brine shrimp, Daphnia, and mosquito larvae. A varied diet supports colouration, condition, and readiness to spawn.
Mating
Like other members of the genus, Hemigrammus coeruleus is an egg-scattering, open spawner with no parental care. Males in spawning condition can be distinguished from females by their slimmer body profile; gravid females appear noticeably deeper-bellied when viewed from above or from the side.
Courting males chase and display to females among fine-leaved vegetation, using rapid pursuit and lateral displays. Spawning occurs as the pair or trio release eggs and milt simultaneously, with adhesive eggs falling into or clinging briefly to vegetation and substrate. Adults pay no attention to the eggs after spawning and will consume them if given the opportunity.
Breeding
Hemigrammus coeruleus has been bred in aquaria. Hobbyist accounts record clutches of roughly 100–150 eggs, with the eggs described as sensitive to light during incubation. A separate, dimly lit breeding aquarium with clumps of fine-leaved plants such as Java moss or synthetic spawning mops is the standard approach. Conditioning the pair on live and frozen foods before moving them to the breeding tank improves spawn frequency and egg count.
Breeding temperature is reported around 81 °F with a pH near 7.5. After spawning the adults should be removed to prevent predation on the eggs. The eggs hatch in a few days; fry are free-swimming within another day or two and initially require very fine foods such as infusoria or commercial liquid fry food before accepting newly hatched brine shrimp. No parental care of any kind is provided.
In the aquarium
Hemigrammus coeruleus is a peaceful, active schooling fish suited to community aquaria. It is best kept in groups of six or more individuals, which encourages natural shoaling behaviour and reduces inter-individual stress. The species does well in a planted aquarium with a dark, fine substrate, open mid-water swimming space, and some shaded areas created by floating plants or tall stem plants at the back.
FishBase records a preferred temperature around 77 °F and pH near 7, pointing to neutral, warm water as a baseline. Given the species' range spanning both whitewater Solimões and blackwater Negro drainages, it likely tolerates moderate variation in water chemistry. Good filtration, regular water changes, and avoiding sudden parameter shifts are more important than hitting precise numbers. It makes a good community fish alongside other small, peaceful tetras, rasboras, small corydoras, and non-aggressive dwarf cichlids.
Conservation
Hemigrammus coeruleus is assessed as Least Concern on the IUCN Red List, with the assessment published in March 2021 and conducted by Flávio Lima of Universidade Estadual de Campinas. The Least Concern designation reflects the species' presence across a broad section of the Amazonian lowlands — both the Solimões and lower Negro basins — without evidence of rapid population decline.
The Amazon basin, while still the world's largest tropical river system, faces increasing pressure from deforestation, agricultural run-off, mercury contamination from gold mining, and large-scale hydroelectric development. These diffuse threats affect many Amazonian fish species simultaneously. For H. coeruleus, no targeted conservation programmes are known, and the species is not listed under CITES. Aquarium specimens are typically wild-caught or bred by hobbyists; supporting captive-bred stock where available reduces pressure on wild populations.