Taxonomy & naming
Boraras merah was described by Maurice Kottelat in 1991 from material collected in the Sungei Jelai Bila drainage, Kalimantan Tengah, Indonesian Borneo. The type locality lies approximately 19 mi south of Sukamara at roughly 2°41'S, 111°13'E — a lowland peat-swamp river in the heart of Sundaland.
The genus Boraras was carved out of the sprawling old genus Rasbora by Kottelat and Vidthayanon in 1993, two years after this species was named. The diagnosis rested on the extreme miniaturisation of the skull and the highly derived arrangement of jaw bones, so Boraras merah was a founding member of the new genus despite being described under a different framework. The Catalog of Fishes (Eschmeyer, CAS) records the valid combination as Boraras merah (Kottelat, 1991), with the parenthetical authority reflecting the transfer from the original placement. The common name phoenix rasbora references the red colouration; merah is the Malay and Indonesian word for 'red', applied directly as the specific epithet.
Boraras sits within the tribe Rasborini (family Cyprinidae) alongside other miniaturised Sundaland rasboras. The genus contains only a handful of described species, all from peat-swamp and blackwater systems of the Thai-Malay Peninsula, Sumatra, Borneo and adjacent islands.
Morphology
Boraras merah is a micro-cyprinid, reaching a maximum of 1 in in standard length — among the tiniest vertebrates kept in the hobby. The body is laterally compressed and nearly ovoid, giving the fish a rounded profile quite unlike the torpedo shape of larger danionins.
The coloration is the species' defining feature: a deep red to orange-red ground colour is overlaid by one or two dark lateral blotches or a broken horizontal stripe of near-black, and the fins carry red pigment with variable dark edging. The patterning intensifies when the fish is in good condition and appropriate blackwater chemistry. Males in breeding condition are especially vivid and are noticeably slimmer than females; females fill out appreciably when gravid, becoming rounder in the abdomen. Both sexes are translucent enough that internal organs are faintly visible through the flanks.
Habitat
The species is endemic to southern Borneo, specifically the Jelai Bila river basin of Kalimantan Tengah. Its natural habitat is the interior of ancient lowland peat-swamp forest — one of the most distinctive and chemically extreme freshwater environments on Earth. Water flowing through compressed peat releases humic and fulvic acids that stain it the colour of strong tea or dark beer, drop the pH to values between 4.0 and 6.5, and strip out almost all dissolved minerals. Conductivity in such waters can fall below 20 µS/cm and total dissolved solids approach zero.
The forest floor is carpeted in leaf litter, submerged root networks and waterlogged wood. Current is gentle to negligible in most peat-swamp channels. Temperatures range from roughly 68–82 °F, tracking ambient air temperature beneath the forest canopy. This combination — ultra-soft, ultra-acidic, tannin-black water at moderate tropical temperatures — is among the most specialised freshwater conditions a hobbyist can be asked to replicate, and it is non-negotiable for the long-term health of Boraras merah.
Feeding
Boraras merah is a micropredator. In the wild it takes tiny invertebrates from the water column and near the surface: small insects and their larvae, microcrustaceans, zooplankton, and aquatic worms. Its minute gape sets an absolute upper size limit on prey items — anything a typical flake fragment would challenge is too large.
In the aquarium, suitably sized live and frozen foods are essential for good condition and colouration: newly hatched Artemia nauplii, Daphnia, microworms, grindal worms, and similar microfoods are accepted eagerly. High-quality micro-pellets and finely crushed flake can supplement but should not form the bulk of the diet. Feeding several small meals per day suits its active, continuous foraging style. Because the fish is so small, it can easily be outcompeted at feeding time, which is another reason species-appropriate tankmates are important.
Mating
Boraras merah is an egg-scattering, non-guarding spawner. When a group is in good condition, spawning activity is essentially continuous — pairs or small groups engage in brief chasing sequences among fine-leaved plants, mosses or root systems, releasing small numbers of adhesive eggs during each pass. There is no pair bond, no territory, and no nest structure of any kind.
A driving male pursues a gravid female closely through vegetation, and the two press together momentarily as eggs and milt are released simultaneously. The adhesive eggs attach loosely to plant surfaces, moss fibres or settle into leaf litter on the substrate. The adults take no interest in the eggs once spawning is complete and will consume them if they encounter them — the same opportunistic micropredator behaviour that governs their feeding applies equally to their own spawn.
Breeding
In a well-maintained blackwater aquarium, Boraras merah spawns daily when conditioned on live and frozen foods, making it a continuous rather than seasonal breeder under aquarium conditions. For deliberate breeding, a small, densely planted tank with a layer of fine moss (Java moss or similar) over the substrate is the standard approach. Soft, acidic water — pH 5.0–6.5, near-zero hardness — is essential; the eggs are unlikely to hatch properly in harder or more alkaline conditions.
Eggs hatch in approximately two days at typical temperatures. The larvae are tiny and initially require infusoria or commercial first foods of equivalent particle size before graduating to Artemia nauplii after several days. Transferring adults to a separate tank after a spawning session, or using a fine mesh base so eggs fall beyond the adults' reach, improves fry survival. Growth is slow by cyprinid standards — the fish's extremely small adult size means the fry remain fragile for longer than larger species.
In the aquarium
Boraras merah is not a beginner's fish, despite its small footprint. Its demands on water chemistry — soft, acidic, tannin-stained blackwater — and its incompatibility with the standard community tank make it a species for keepers who can dedicate a species-appropriate setup. A tank of 10–15 US gal with a dark substrate, dense planting of fine-leaved plants, floating vegetation, driftwood and dried leaves, and a sponge or matten filter providing slow, gentle flow is the target environment.
A school of at least eight to ten individuals is strongly recommended. In smaller numbers the fish become timid and spend excessive time hiding; a proper school schools visibly, moves more confidently, and displays better colouration. Suitable companions — if any are kept — must be peaceful, similarly small and equally demanding of soft acidic water: other Boraras species, dwarf Corydoras, small Otocinclus, and diminutive anabantoids such as Sphaerichthys or young Betta species in appropriate setups. Standard community fish, even small tetras from harder water, are unsuitable. The species is sensitive to water quality swings; stable chemistry and careful acclimatisation are more important than hitting exact parameter targets.
Conservation
Boraras merah is assessed as Data Deficient (DD) on the IUCN Red List, most recently evaluated in 2019 with the assessment remaining unchanged in the 2025-1 update. The Data Deficient listing reflects genuine uncertainty about the species' area of occupancy and the trend of its population — not an absence of concern. The threats to its habitat are severe and well-documented: the peat-swamp forests of Kalimantan Tengah have been extensively cleared and drained for palm oil agriculture, timber extraction and transmigration settlements. Peat-swamp drainage is an acute threat because it both destroys the physical habitat and alters the hydrology and chemistry of surrounding water bodies permanently. Additional pressures include collection for the aquarium trade, though captive breeding has substantially reduced demand for wild-caught specimens.
The fragmented and degraded state of Bornean peat-swamp forest means that a species endemic to it — particularly one with limited dispersal ability and extreme water-chemistry requirements — faces a genuinely uncertain future, regardless of its formal listing status. Captive-bred fish are widely available and should be preferred by hobbyists.