Taxonomy & naming
Rasbora einthovenii was formally described by the prolific Dutch ichthyologist Pieter Bleeker in 1851, originally from material collected in Borneo. The Catalog of Fishes (Eschmeyer, the authority for valid names) records the valid combination as Rasbora einthovenii (Bleeker, 1851), with the parenthetical authority indicating the species was originally placed in a different genus before settling in Rasbora.
The genus Rasbora has been substantially restructured since the 1990s. Kottelat and Witte (1999) erected Trigonostigma, Microrasbora and other genera to absorb taxa previously lumped into Rasbora, and subsequent workers carved out Boraras, Trigonopoma, Brevibora, Kottelatia and Rasboroides. The harlequin rasboras and many dwarf species have migrated accordingly. Rasbora einthovenii itself has remained in Rasbora throughout these revisions; GBIF records it within the subfamily Danioninae of the family Cyprinidae, the broad grouping that also contains the true danios. It is not to be confused with any of the recently reclassified dwarf or harlequin-type rasboras.
The species epithet honours Wynand Eindhoven (also rendered Eindhoven), a Dutch physician and naturalist who worked in the Dutch East Indies and supplied specimens to Bleeker.
Morphology
Rasbora einthovenii is a moderately large rasbora, reaching up to 3.5 in SL (standard length) and occasionally recorded to around 4 in SL in the wild — stocky for the genus. The body is fusiform and moderately compressed, with a slightly pointed snout and a terminal to slightly upturned mouth adapted to surface and mid-water feeding.
Colouration is the source of the common name: the flanks carry a bright, often iridescent lateral stripe — typically silver-gold or bluish — that runs from behind the gill cover to the caudal fin base, contrasting with an olive to bronze dorsum and a paler, whitish underside. The fins are generally hyaline (clear) to faintly pigmented. Colouration can intensify in blackwater conditions and in well-conditioned fish.
Sexual dimorphism is moderate. Gravid females become noticeably deeper-bellied and rounder through the abdomen, whereas males are slimmer and may show more intense flank colouration during courtship. Differences are most apparent when a mixed group is compared side by side.
Habitat
Rasbora einthovenii is native to the Malay Peninsula (including southern Thailand, Peninsular Malaysia and Singapore) and the Greater Sunda Islands — Borneo and Sumatra — with additional records from Bangka Island. Across this range it inhabits blackwater streams, peat-swamp forest rivers and slow-flowing lowland watercourses that run through intact or remnant forest.
The defining chemical character of its habitat is extreme softness and acidity: water draining through peat swamps is stained dark brown with tannins and humic acids, with pH values that may fall well below 5 in pristine forest streams and hardness close to zero. Seriously Fish reports field conditions of 72–79 °F, pH 5.0–7.5 and hardness of 2–10°H for the species. In practice the lower end of that pH range is typical of its core peat-swamp habitat, and fish kept in harder, more alkaline water often fare poorly long-term.
Within such streams the species is found in mid-water to surface layers, typically in groups, in areas sheltered from strong current by root tangles, overhanging vegetation and fallen timber. It is often sympatric with other small rasboras, halfbeaks and small labyrinth fishes.
Feeding
Rasbora einthovenii is a micropredator in the wild, targeting small invertebrates available at the surface and in the water column. FishBase records its natural diet as worms, crustaceans and insects, and Seriously Fish confirms it as a micropredator taking small insects, worms, crustaceans and zooplankton — a typical profile for a mid-sized Sundaic rasbora hunting in the soft, food-dilute waters of a peat-swamp stream.
In the aquarium it is an eager feeder that accepts high-quality dried foods, but colour and condition are best maintained with regular offerings of small live and frozen invertebrates: daphnia, mosquito larvae, bloodworm and brine shrimp are all suitable. Given its size relative to many other rasboras, it can also take somewhat larger prey items. Feeding at the surface and through the water column, it is active throughout the day.
Mating
Rasbora einthovenii is an egg-scattering, non-guarding spawner. No nest is built; there is no bubble-nest, no mouthbrooding and no guarding of eggs or fry. Courtship follows the typical rasbora pattern: males in breeding condition chase gravid females through dense vegetation or over submerged roots, pressing close alongside them, and eggs and milt are released together during brief embraces among the plants.
The pair bond lasts only for the moment of spawning. Eggs are scattered among fine-leaved plants, moss and substrate debris, where they may be eaten by adults if both sexes are left in the same water. Spawning in the wild is likely linked to the seasonal rains and warming that characterise lowland Sundaic streams, triggers that can be partially replicated in captivity.
Breeding
Breeding Rasbora einthovenii in captivity follows the standard approach for soft-water Sundaic rasboras. A separate spawning tank of 15 US gal or more, filled with very soft, acidic, lightly tannin-stained water (pH 5.5–6.5, hardness near zero, temperature around 79 °F) and planted with fine-leaved species such as Java moss or Ceratophyllum gives the best results. A gentle sponge filter maintains water quality without creating current strong enough to scatter eggs.
Well-conditioned adults are introduced in a ratio of two males to one female, or as a small group. The fish scatter adhesive eggs among the plants and over the substrate over the course of a morning; adults should be removed once spawning is complete, as they will eat the eggs. Eggs hatch in roughly 24–36 hours at 79 °F, and the larvae become free-swimming within a few days, when they can be fed infusoria, green water and later newly hatched brine shrimp nauplii. Growth is steady in soft, clean water with frequent small water changes.
In the aquarium
Rasbora einthovenii is best suited to a specialist Sundaic biotope or soft-water community aquarium. Seriously Fish stresses that it is very peaceful and gregarious, and a group of at least six — preferably ten or more — is needed for the fish to be confident and display natural behaviour; smaller numbers result in timid, stressed individuals that hide and fail to colour up. Suitable companions are other small to medium soft-water species: small rasboras, pencilfish, dwarf gouramis, loaches from Sundaland, and non-aggressive bottom-dwellers.
Water chemistry is non-negotiable. The species struggles in harder, more alkaline water — the common tap-water chemistry of most temperate countries — and long-term maintenance requires reverse-osmosis water or a very soft local supply, acidified and tinted with peat or dried Indian almond leaves. A temperature of 72–79 °F, dim lighting and a dark substrate enhance colour and reduce stress. Dense planting, driftwood roots and leaf litter recreate the tannin-rich forest-stream environment the fish needs.
Fed well and kept in appropriate water, the brilliant rasbora is a handsome, active display fish for a dedicated soft-water set-up. It rewards the effort required to replicate its chemistry with good health, bright colour and relaxed schooling behaviour.
Conservation
Rasbora einthovenii is assessed as Least Concern (LC) on the IUCN Red List (2024). The species has a reasonably broad range across the Malay Peninsula, Sumatra and Borneo, and remains locally common where forest-stream and peat-swamp habitat is intact. Its wide distribution prevents a range-wide collapse even as individual localities disappear.
Nonetheless, the threats to its habitat are serious and accelerating. Peat-swamp forests across Sundaland have been extensively drained, burned and converted to oil-palm and acacia plantations over recent decades, removing the very ecosystem that defines this fish's water chemistry and cover. Water pollution from agriculture and industry further degrades remaining streams. The IUCN notes habitat destruction, water pollution and overfishing as active threats in the native peat-swamp range. Singapore, once part of the species' range, has lost virtually all of its peat-swamp forest and the species is locally extinct or extremely rare there. The Least Concern designation reflects current population size, not immunity to future decline; continued deforestation in Malaysia, Indonesia and Thailand makes ongoing monitoring important.