Taxonomy & naming
Trigonostigma hengeli was described by Hermann Meinken in 1956. It was originally placed in the large catch-all genus Rasbora and known in the hobby for decades as Rasbora hengeli. The genus Trigonostigma was erected by Kottelat & Witte in 1999 to accommodate a small cluster of harlequin-type rasboras — characterised by their leaf-spawning reproductive strategy and the distinctive dark triangular or wedge-shaped lateral marking — and T. hengeli was transferred at that time. The Catalog of Fishes (Eschmeyer, CAS) records the current valid combination as Trigonostigma hengeli (Meinken, 1956), the parenthetical authority marking the move from the original genus.
The genus contains only a handful of species, of which T. heteromorpha (the classic harlequin rasbora), T. espei, and T. hengeli are the three most familiar to aquarists. Trigonostigma hengeli and T. espei are sometimes confused: both are slimmer and more lightly built than the broad-bodied harlequin, and both carry an orange-copper stripe above a fine dark wedge mark. The species epithet hengeli honours Gustav Hengel, the German aquarist who first imported living specimens to Europe.
Morphology
The glowlight rasbora is a small, slender, laterally compressed cyprinid reaching around 1 in standard length at maturity. The body profile is more elongate and less deep than its congener T. heteromorpha, giving the fish a lighter, more torpedo-like silhouette. The most striking feature is a narrow but intensely pigmented stripe that runs along most of the upper flank — a vivid copper-orange to rust-red that glows almost neon-like under indirect aquarium light, the source of the common name.
Below this stripe, and particularly in the posterior half of the body, lies a fine dark wedge marking that converges toward the caudal peduncle — more attenuated and less bold than the solid black triangle of the harlequin rasbora. Fins are largely hyaline with a faint reddish tinge. Sexual dimorphism is mild: females are slightly deeper-bellied and paler when gravid; males remain slimmer and may display more intensely when courting.
Habitat
Trigonostigma hengeli is native to the Greater Sunda Islands — Sumatra and Borneo — where it inhabits slow-moving forest streams, blackwater pools, and the margins of peat-swamp drainage systems. The type locality is the Tambesi river in Jambi Province, Sumatra, a tributary of the Batang Hari system. Natural habitats are characterised by extremely soft, weakly acidic to mildly neutral water stained amber to dark brown by humic and tannic compounds leaching from leaf litter and submerged peat; dissolved mineral content is very low.
Light penetration in these swampy forest streams is dim to moderate, filtered through dense canopy. Substrates are typically dark peat or fine silt covered in decomposing leaves, and Cryptocoryne species are frequent aquatic plants in the same zones where this fish is found in the wild. Water temperatures reflect the equatorial lowland climate — consistently warm across the year, typically in the 73–82 °F range.
Feeding
Trigonostigma hengeli is a micropredator in the wild, feeding principally on tiny invertebrates — zooplankton, small aquatic insect larvae, and other minute organisms at or near the surface of the water column. The small gape and delicate build of the fish reflect a diet composed largely of very fine live prey items rather than larger morsels.
In the aquarium it accepts a broad range of small foods readily: quality fine-grade flake and micro-pellets make an adequate staple, but condition and colour are best sustained with regular offerings of small live and frozen foods such as daphnia, baby brine shrimp, micro-worms, and bloodworm (chopped or of suitably small size). Because of its small mouth, foods sized for larger cyprinids may be ignored or rejected.
Mating
Trigonostigma hengeli follows the characteristic reproductive strategy of its genus — a leaf-spawning mode that is unusual among rasboras and distinguishes Trigonostigma from the typical egg-scattering behaviour of most danionin cyprinids. Rather than broadcasting eggs freely over the substrate or through vegetation, the pair selects a spawning site on the smooth underside of a broad, horizontal leaf — in the wild most often a Cryptocoryne leaf, in the aquarium any suitable broad-leaved plant.
Courtship involves persistent driving by the male, who pursues the female through the planting and alongside favoured leaves. When the female is ready, the pair aligns belly-to-belly in an inverted position beneath the chosen leaf and releases a small cluster of adhesive eggs directly onto its surface. The sequence is repeated over the course of a morning spawning session, the pair visiting the same or additional leaves multiple times. No pair bond forms beyond the spawning event, and neither sex guards the eggs or territory afterward.
Breeding
A breeding setup for T. hengeli should aim to replicate the species' native blackwater conditions as closely as practical. A small, densely planted tank with soft, acidic water (pH 5.5–6.5) and very low hardness, dimly lit and decorated with broad-leaved plants — ideally Cryptocoryne species — provides the necessary environment. Conditioning the fish on small live and frozen foods for a week or two before the attempt noticeably improves readiness.
Spawning typically occurs in the morning hours. Eggs are deposited in small batches on the undersides of leaves, each batch numbering only a handful, and the total spawning yield is modest compared with free-scattering species. Incubation is approximately 24–48 hours at 79–82 °F, after which tiny larvae emerge. Because the adults give no parental care and will consume eggs and fry if present, the spawning group is best removed after the session concludes, or the eggs transferred on the leaf section. Larvae begin free-swimming a few days after hatching and require infusoria or similar microscopic first foods, progressing quickly to nauplii of brine shrimp.
In the aquarium
The glowlight rasbora is a peaceful, undemanding schooling fish well suited to smaller, heavily planted community aquaria with other calm, fine-water species. Its small adult size — under 1 in — means it can be housed in tanks from around 23.5 in in length, but a school of at least eight to ten individuals is necessary for the fish to behave naturally and display confidently. In numbers they form a fluid, constantly moving group that makes an elegant centrepiece in a Southeast Asian blackwater biotope.
Suitable companions include other soft-water species of similar disposition: small Boraras, Trigonopoma, Rasbora species of comparable size, pygmy Corydoras, and peaceful Kuhli loaches share similar water requirements. The water chemistry is the most demanding aspect of the husbandry: T. hengeli is best kept in soft (hardness well below 100 ppm), warm (73–82 °F) and slightly acidic to neutral (pH 5.0–7.5) water and does not thrive long-term in hard alkaline conditions typical of many tap supplies. Careful acclimatisation is advisable for wild-caught or delicate captive stocks. Subdued lighting and a natural-toned decor of dark substrate, driftwood and leaf litter best show the copper-orange stripe to advantage.
Conservation
Trigonostigma hengeli is assessed as Least Concern (LC) on the IUCN Red List. It has a relatively broad distribution across the Sundaic peat-swamp and forest-stream zone of Sumatra and Borneo, it is common in the aquarium trade, and the majority of fish reaching hobbyists are commercially bred rather than wild-caught.
Nonetheless, the habitats this species occupies — lowland peat swamps and forest streams in Sumatra and Borneo — are among the most rapidly deteriorating freshwater environments in Southeast Asia. Large-scale drainage for palm oil agriculture, peat fires, logging, and the conversion of swamp forest to plantation have destroyed or severely degraded vast areas of Sundaic blackwater habitat in recent decades. While T. hengeli's populations have not collapsed to the point warranting a threatened listing, the ongoing loss of peat-swamp ecosystems across its range remains a real and escalating pressure on the long-term security of wild populations.