Taxonomy & naming
Rasbora dusonensis was described by Pieter Bleeker in 1850, originally from material associated with the Duson River drainage in southern Borneo (Banjarmasin area, Barito drainage). The Catalog of Fishes (Eschmeyer) records the valid combination as Rasbora dusonensis (Bleeker, 1850), with the parenthetical authority reflecting the shift from Bleeker's original genus placement.
The genus Rasbora has been substantially revised over the past three decades: genera including Boraras, Trigonopoma, Brevibora, Kottelatia and Rasboroides were carved out of it, and numerous former 'rasboras' have been moved to Trigonostigma or other newly erected genera. Rasbora dusonensis itself remains in Rasbora sensu stricto under current classifications and is part of the core Asian rasbora assemblage within the family Cyprinidae.
FishBase lists 'rosefin rasbora' as the primary English common name; 'yellowtail rasbora' is also encountered in the aquarium trade and in older literature.
Morphology
Rasbora dusonensis is a relatively large rasbora, typically reaching 3.5–4 in standard length in the wild, with exceptional individuals recorded to approximately 6.5 in SL. The body is elongate and laterally compressed in the typical rasbora plan, with a pointed snout, a terminal or slightly superior mouth, and a forked caudal fin.
The flanks are silver, often with a faint golden or yellowish cast along the lateral line region and toward the tail — the feature that gives the fish its 'yellowtail' trade name. The fins may show a rosé or orange tint in life, particularly in the dorsal and caudal fins of well-conditioned individuals, which accounts for the alternative common name 'rosefin rasbora'. The back is olive to brown and the belly pale silver-white.
Sexual dimorphism in this species, as in most large rasboras, is subtle outside of breeding condition: females become noticeably fuller-bodied and rounder in the abdomen when gravid.
Habitat
Rasbora dusonensis is native to slow-flowing blackwater streams, peat-swamp river channels and associated river systems across a broad Sundaic range: Sumatra (Indragiri and Riau drainages), Peninsular Malaysia, and Borneo (including the Barito drainage of South Kalimantan, the type locality). Some clear-water occurrences have also been recorded, suggesting the species can tolerate a broader spectrum of lowland freshwater environments than the strict peat-swamp specialists.
The defining character of its core habitat is soft, acidic, tannin-stained water: Seriously Fish records a pH range of 4.0–7.5 and temperatures of 68–79 °F, with hardness varying from very soft to moderately hard (18–215 ppm). The substratum is typically composed of organic matter — fallen leaves, waterlogged wood and branches — with little or no mineral sediment, and canopy shading keeps light levels low. These conditions are characteristic of Sundaland peat-swamp forest, one of the most threatened habitat types in Southeast Asia.
Feeding
In the wild, Rasbora dusonensis feeds primarily on aquatic and terrestrial invertebrates: insects and their larvae, small crustaceans and other invertebrates that fall from riparian vegetation or drift in the current. The relatively large body and active swimming style suit a mid-water to surface feeder capable of pursuing mobile prey.
In the aquarium, the species requires meaty foods to maintain condition. Live and frozen items — Daphnia, Artemia, bloodworm and similar — should form a significant portion of the diet. The fish readily accepts quality dried foods once settled, but a diet composed entirely of flake or pellet food is unlikely to bring it to peak colour and condition. As with most larger rasboras, feeding once or twice daily in amounts consumed within a few minutes is appropriate.
Mating
Rasbora dusonensis is an egg-scattering, non-guarding spawner. Like the great majority of cyprinid rasboras, it forms no lasting pair bond and constructs no nest. In the wild, spawning is presumed to be tied to seasonal cues — rainfall and rising water levels in its peat-swamp and river-channel habitats — with receptive females pursued by driving males through vegetation and along submerged woody debris.
During courtship the male presses close to the female's flank, and eggs and milt are released in brief bursts over plants, moss or the substrate. The eggs receive no attention from either parent after being scattered, and in an aquarium setting the adults will readily consume both eggs and larvae if given access.
Breeding
Aquarium breeding of Rasbora dusonensis has not been confirmed in published accounts, and the species appears rarely in breeding reports — a reflection of its relative scarcity in the hobby and the demanding conditions it likely requires. Based on what is known of close relatives and the species' blackwater origins, any breeding attempt would need to replicate very soft, acidic, warm water closely: pH in the range of 4.5–6.5, low hardness, and temperatures toward the upper end of its tolerance (75–79 °F).
A dedicated spawning aquarium with fine-leaved plants or spawning mops, shaded from direct light, would provide sites over which the egg-scattering pair could spawn. Eggs are reported to hatch within 18–48 hours, with fry becoming free-swimming after a further 24–48 hours; first foods would be infusoria or very fine commercial fry foods, advancing to newly hatched brine shrimp as the young grow. As with all egg-scattering rasboras, parents should be removed immediately after spawning.
In the aquarium
Rasbora dusonensis is an active, schooling fish that is best kept in groups of six or more individuals; solitary or paired specimens become timid and show poorly. Its eventual size — routinely exceeding 4 in — means it requires a spacious aquarium: a minimum footprint of 47 in length is a reasonable starting point for a small school, with more space preferred. Tankmates should be of comparable size and temperament; small fishes risk being startled by the fast-moving rasboras, and very small species could theoretically be treated as food.
Water quality is the primary keeper challenge. The species originates from soft, acidic, often peaty water, and while it can tolerate a fairly broad pH range (4.0–7.5 per field data), it displays best colour and condition at the softer, more acidic end. Temperatures of 72–79 °F are appropriate. Décor of driftwood, dried leaf litter and low-light plants such as Java fern replicates the natural environment and helps keep the fish settled. Strong aeration and good filtration are essential, as the slow-flowing blackwater habitats the fish occupies in the wild are typically well-oxygenated despite their dark appearance.
Conservation
Rasbora dusonensis is assessed as Least Concern (LC) on the IUCN Red List, a reflection of its wide distribution across multiple major drainages — from Sumatra through Peninsular Malaysia to Borneo — and the absence of evidence of significant population-level decline across that range.
Nevertheless, the peat-swamp forests and lowland river systems that form its core habitat in Sundaland are among the most threatened freshwater environments in the world. Drainage for agriculture (principally oil palm), logging, peat fires and pollution have destroyed or degraded vast areas of lowland Bornean and Sumatran habitat over the past several decades, and continue to do so. Species with a wide enough range to span multiple catchments are buffered against local losses, but the long-term trend of habitat loss across Sundaland remains a concern for the freshwater fauna as a whole.