Taxonomy & naming
Rasbora borapetensis was described by the American ichthyologist Hugh McCormick Smith in 1934 from specimens collected at Bung Borapet (also spelled Bueng Boraphet), a large freshwater lake and surrounding floodplain in Nakhon Sawan Province, central Thailand. The species epithet borapetensis directly commemorates this type locality.
The genus Rasbora, as currently recognised by the Catalog of Fishes (Eschmeyer, CAS — the authority for valid names), retains R. borapetensis. The traditional genus Rasbora is broadly paraphyletic, and since the 1990s numerous genera have been split from it — Boraras, Trigonopoma, Brevibora, Trigonostigma, Rasboroides and others — reshaping the subfamily Danioninae considerably. Rasbora borapetensis has not been reassigned in any of these revisions and remains in the nominotypical genus.
In the hobby it is occasionally confused with R. pauciperforata (red-stripe rasbora), which shares the dark lateral band, or with R. elegans. Smith's 1934 description is lodged in the primary literature; subsequent treatment in FishBase and standard reference works has stabilised the name.
Morphology
Rasbora borapetensis is a slim, laterally compressed cyprinid that typically reaches about 2 in standard length in the aquarium, with FishBase recording a maximum of 4 in total length for large wild specimens. The body plan is classic rasbora — torpedo-shaped, with a forked caudal fin, 9 dorsal soft rays and 8 anal soft rays.
The most distinctive feature is a sharp, dark horizontal stripe running from behind the eye to the base of the caudal fin along the midline of the flank. Above this stripe the flanks are silvery-olive, below it paler silver. The caudal, dorsal and sometimes the anal fins carry a reddish or orange tint, particularly in well-conditioned fish, which has led to the alternative common name red-tailed rasbora. The mouth is small and slightly upturned, consistent with a surface-to-mid-water feeding habit.
Habitat
The blackline rasbora inhabits the lowland freshwaters of mainland Southeast Asia — the Mekong, Chao Phraya and Meklong river basins in Thailand, and likely Laos, Cambodia and Vietnam as drainage divides permit, as well as the northern Malay Peninsula. Its type locality, Bung Borapet in Nakhon Sawan Province, is one of Thailand's largest natural freshwater lakes: a shallow, floodplain reservoir surrounded by seasonally inundated ricefields, ditches and canals — precisely the habitat this species favours.
Field and aquarium literature consistently describe it in shallow, gently flowing or still waters: ponds, irrigation ditches, reservoirs and the vegetated margins of slow rivers. Water conditions at these lowland sites are soft to moderately hard (up to ~12° dH), with a slightly acidic to neutral pH in the range of 6.0–7.5, and temperatures of 72–79 °F. This is not a peat-swamp blackwater species and does not require the extreme acidity of Sundaland endemics, but it is equally at home in the slightly soft, slow, warm waters of the Thai floodplain.
Feeding
Rasbora borapetensis is an omnivore with a strong predisposition toward small live prey. FishBase records zooplankton, insects, worms and small crustaceans as wild dietary items — a diet typical of a mid-water schooling cyprinid hunting by sight in still or slow water.
In the aquarium, Seriously Fish and hobbyist consensus note that the species strongly prefers small live and frozen foods — Daphnia, brine shrimp (Artemia nauplii or adults) and similar items. It will accept dry foods but does not show the same enthusiasm, and condition, colour intensity and willingness to spawn all benefit from regular live or frozen supplement. Small, frequent offerings suit the species better than a single large daily feed.
Mating
Rasbora borapetensis is an egg-scattering, non-guarding spawner. Like the great majority of danionin cyprinids, it forms no pair bond, builds no nest and provides no care beyond the moment of fertilisation. In a conditioned group a ripe male will pursue a gravid female persistently through the vegetation, pressing close alongside her as eggs and milt are released simultaneously.
Spawning activity in the aquarium tends to occur in the morning. Adults take no further interest in the eggs and, given the opportunity, will consume them readily. Separating adults from eggs as soon as spawning is complete — or using a mesh or dense plants to allow eggs to fall out of reach — is the standard approach.
Breeding
Captive breeding is straightforward. A separate spawning vessel with fine-leaved plants, Java moss or a nylon spawning mop is standard, with a mesh or marbles on the base to protect eggs that fall clear of the vegetation. Water should match the species' preferred parameters — 75–79 °F, pH 6.0–7.0, soft to moderately soft. Conditioning with Daphnia and brine shrimp over several days before introduction accelerates readiness.
A productive spawning yields 30–50 eggs per session according to hobbyist accounts, with eggs hatching within 18–48 hours and the fry becoming free-swimming a further 24–48 hours after hatching. First foods are infusoria and green water, transitioning rapidly to newly hatched brine shrimp as the fry grow. Growth is steady at tropical temperatures and the species matures in a few months. There is no parental care of any kind.
In the aquarium
The blackline rasbora is a hardy, community-friendly fish well suited to a beginner or intermediate keeper working with Southeast Asian biotope setups. Its natural habitat of slow, warm, vegetated lowland waters translates directly to the aquarium: a well-planted tank with a dark substrate, some driftwood for tannin-tint and slow surface movement suits it well. Water should be soft to moderately hard (up to ~12° dH) and slightly acidic to neutral (pH 6.0–7.5) at 72–79 °F.
The species is a confirmed schooler. Groups of at least 8–10 individuals display markedly better behaviour and bolder activity than smaller numbers; a lone or small group rasbora spends much of its time hiding. In a proper school the fish engage in the constant, animated interaction typical of the family — circling, light chasing and the glinting flash of turning flanks. It is peaceful with similarly sized tankmates and not a fin-nipper. Compatibility with other slow-water Southeast Asian fishes (gouramis, small loaches, other rasboras) is excellent.
Conservation
Rasbora borapetensis is assessed as Least Concern (LC) on the IUCN Red List (2020 assessment). Its broad distribution across the major river basins of mainland Southeast Asia — the Mekong, Chao Phraya and Meklong systems, and the northern Malay Peninsula — combined with its tolerance of disturbed, modified habitats such as irrigation ditches, canals and reservoirs, gives it a resilience that more specialised species lack.
The species is collected for the ornamental trade but is also bred commercially in sufficient quantities that wild-sourced pressure is not considered a primary threat. Ongoing habitat pressures in lowland Southeast Asia — agricultural intensification, irrigation infrastructure, water quality degradation from pesticide and fertiliser runoff, and floodplain modification — represent the background stressors on populations across the range, though none currently drives a population decline meeting threatened thresholds.