Taxonomy & naming
Boraras micros was described by Maurice Kottelat and Chavalit Vidthayanon in 1993, the same paper that erected the genus Boraras to accommodate a cluster of very small rasbora-like cyprinids previously placed in Rasbora. The name Boraras is an anagram of Rasbora, reflecting the genus's origin as a segregate from that large, heterogeneous assemblage. The Catalog of Fishes (Eschmeyer, CAS) records the valid combination as Boraras micros Kottelat & Vidthayanon, 1993.
The genus Boraras is endemic to Sundaland and adjacent mainland Southeast Asia, comprising a handful of species united by their exceptionally small adult size, large eyes relative to body length, and preference for soft, acidic blackwater habitats. Within this group, Boraras micros is the smallest species, its epithet reflecting that distinction directly. It should not be confused with Boraras urophthalmoides or Boraras maculatus, which are larger, more frequently traded, and carry different lateral-line patterns.
Morphology
Boraras micros reaches a maximum of roughly 0.5 in standard length, making it among the tiniest cyprinids kept in the hobby. The body is slender and translucent, the flanks pale with a faint pinkish or orange flush in well-conditioned individuals. A dark lateral stripe or spot series runs along the midflank, though the precise pattern is less bold than in some congeners. The fins are small and hyaline, the eyes proportionally large — a common feature in miniaturised fishes that must resolve fine detail in dim, turbid swamp water.
FishBase records a maximum size of 0.5 in SL. Sexual dimorphism is modest but discernible: females become noticeably deeper-bodied when gravid, while males remain more torpedo-shaped and may show marginally brighter colouration. At this scale the fish is easily overlooked in a planted aquarium and is best appreciated in a species-only setting against a dark substrate.
Habitat
The type locality is a swamp approximately 4 mi north of Udon Thani in north-eastern Thailand (17°28′N 102°48′E), within the Mekong drainage. Records cover the provinces of Nong Khai, Udon Thani and Sakon Nakhon in Thailand; the species may also occur in adjacent Laos, though this is unconfirmed. It is a fish of shallow, densely vegetated swamps, marshes and the flooded margins of rice paddies — still or barely moving water over soft substrate, heavily colonised by aquatic macrophytes.
Water conditions in these habitats are soft and mildly acidic, the chemistry shaped by decaying organic matter. Seriously Fish reports a temperature range of 68–82 °F and pH 6.0–7.0, with hardness of 36–179 ppm — conditions typical of lowland Mekong swamp water. The habitat is structurally complex and offers abundant cover, which the species relies on; open, exposed water is rarely occupied.
Feeding
Boraras micros is a micropredator in the wild, feeding on the tiny invertebrates — small insects, microcrustaceans, zooplankton and similar fare — that populate densely vegetated swamp water. Its minute gape constrains it to correspondingly small prey items, and it almost certainly filters a proportion of its food from the water column passively as it moves through vegetation.
In captivity the diet should reflect this: Daphnia, Artemia nauplii, micro-worms, and other live or frozen miniature invertebrates are appropriate staples, supplemented with a quality micro-pellet or fine flake. Suitably sized food is non-negotiable at this scale — standard community-tank fare is often too coarse for the fish to ingest. Feeding in a species-specific or very small-fish tank simplifies competition and ensures the fish actually consume what is offered.
Mating
Boraras micros is an egg-scattering, non-guarding spawner. Like other rasboras and danios, it forms no pair bond and builds no nest; spawning is a brief, repeated act rather than an extended courtship involving territory defence or nest construction. A driving male pursues a gravid female through dense vegetation, pressing alongside her as eggs and milt are released simultaneously among plants or over the substrate.
Spawning in this genus tends to be a continuous low-level process in well-maintained groups rather than a single seasonal event, and the species has been described as a continuous spawner. Adults do not distinguish between their own eggs and potential food, so the eggs receive no parental attention once released.
Breeding
Boraras micros is reported to be rarely bred in captivity, and successful breeding is considered a hobbyist challenge rather than a routine event. When it does occur, conditions must closely replicate the natural habitat: very soft, acidic water (pH around 6.0–6.5), warm temperatures in the low-to-mid twenties Celsius, and a heavily planted tank with fine-leaved vegetation such as java moss or aquatic ferns to receive the scattered adhesive eggs.
Adults must be removed or the eggs separated after spawning, as egg predation by the adults is the norm. Eggs are small and the larvae are tiny on hatching; first foods must be correspondingly fine — infusoria or paramecia initially, moving to Artemia nauplii only when the fry have grown enough to accept them. This scale of care, combined with the species' small commercial profile, accounts for its scarcity in the hobby relative to congeners.
In the aquarium
Boraras micros is best maintained in a species-only aquarium or alongside other very small, non-competitive fishes of similar water requirements — small Dario species, Trigonostigma or Microrasbora species that share its soft-water chemistry, or diminutive invertebrates such as freshwater shrimp. A tank of 10–15 US gal is ample for a group; what matters far more than volume is water quality, density of planting and the soft, acidic chemistry the species requires. Dark substrate, tannin-stained water and subdued lighting will encourage natural behaviour and the best colouration.
Group size matters: this is a schooling fish by nature, and individuals kept in isolation or very small numbers are visibly stressed and pale. A group of at least eight to ten is recommended. The species is intolerant of the harder, more alkaline conditions that suit many community-tank staples, and should never be mixed with active, large-mouthed species that will outcompete it at the food dish or simply eat it.
Conservation
Boraras micros is assessed as Least Concern (LC) on the IUCN Red List, with an assessment date recorded by FishBase as 2011. The species has a restricted known range in north-eastern Thailand, centred on swamp and marsh habitats in the Mekong lowlands — a landscape subject to considerable pressure from agriculture, including the conversion of wetlands to rice paddies and the associated use of pesticides and fertilisers.
The confirmation of the type locality within agricultural lowlands underscores this vulnerability: the very habitat that defines the species is precisely the landscape most transformed by farming in the region. Though LC status reflects no documented collapse, the narrow range and specialist swamp-habitat dependency of Boraras micros warrant ongoing attention, particularly as drainage and agricultural intensification continue across the Mekong basin floodplain of north-eastern Thailand.