Taxonomy & naming
Nemacheilus kapuasensis was described by the Swiss ichthyologist Maurice Kottelat in 1984, with a type locality in the Kapuas River drainage of Kalimantan Barat (West Kalimantan), Indonesian Borneo. It is placed in the family Nemacheilidae, the stone or brook loaches, which was split out of the old, catch-all Balitoridae/Cobitidae complex and is now recognized as its own family and the largest of all the loach lineages, with genera such as Schistura, Barbatula, Oxynoemacheilus and Triplophysa carved from what earlier authors lumped together as Noemacheilus or Nemacheilus.
That history matters for a species like this one: many Southeast and South Asian stream loaches that were once filed under the broad "Nemacheilus" umbrella have since been moved to other genera as the group was revised, and the Catalog of Fishes remains the reference for which combination is currently valid. No record of an old-genus combination for this particular species turned up in the available sources, so it is treated here under its original and current combination, Nemacheilus kapuasensis.
Morphology
As a member of Nemacheilidae, Nemacheilus kapuasensis has the general stone-loach build shared across the family: an elongate, sub-cylindrical body, a small head, and a slim caudal peduncle suited to holding position in current rather than cruising open water. Nemacheilids are near-scaleless or bear only tiny, deeply embedded scales, leaving the skin effectively bare — a trait that matters far more for husbandry than for looks, since it makes the fish sensitive to medications and copper.
Detailed morphometric data specific to N. kapuasensis were not found in the sources consulted here, so exact proportions, fin-ray counts and colour pattern are not asserted; this is a genuinely data-sparse species outside the original description. Where the genus applies it, the erectile suborbital spine typical of some loach lineages should be checked against the primary description rather than assumed present, since not all nemacheilids carry a prominent one.
Habitat
The species is known from forest streams in the Kapuas River drainage of West Kalimantan and from streams in Sarawak, on the island of Borneo. It occurs alongside other small stream specialists — the loach Nemacheilus saravacensis, the burrowing eel-loach Kottelatlimia pristes and the kuhli-loach relative Pangio semicincta — a community typical of shaded, structurally complex lowland-to-hill forest streams on Borneo.
Nemacheilid stone loaches as a family are stream specialists: clear, cool-to-moderate, fast-flowing, well-oxygenated water with low mineral hardness, running over sand, fine gravel and rock, often under a forest canopy. Nothing in the available sources suggests this species departs from that pattern, and there is no indication it tolerates still or warm blackwater conditions in the way many other Bornean fishes do.
Feeding
Nemacheilus species in general are omnivorous micro-predators, foraging over the streambed for small insects, worms and crustaceans, with Daphnia, Artemia and bloodworm-type prey standing in for their natural diet in captivity. Species-specific feeding data for N. kapuasensis were not found, so this picture is drawn from the genus rather than confirmed observation of this fish.
In practice this means a keeper should expect a bottom-feeding hunter rather than a fish that competes well for flake at the surface: sinking live and frozen foods, offered where the fish can find them on the substrate, are the safest bet for meeting its needs.
Mating
No description of courtship, spawning behaviour or sexual dimorphism specific to Nemacheilus kapuasensis was found in the sources consulted. Stone and brook loaches in general show little obvious external dimorphism outside the breeding season, and reproduction in the wild is generally tied to seasonal rises in flow that trigger spawning migrations into shallower stream reaches — but this is a family-level pattern, not a confirmed observation for this species.
Breeding
There is no record of Nemacheilus kapuasensis being bred in the aquarium. Stone and brook loaches of this genus are, as a rule, egg-scatterers: ripe adults release eggs over open gravel or among stones in fast-flowing water, with no parental care given to eggs or fry afterward. Home spawnings of Nemacheilidae are rare across the family, and this species — never established in the hobby and known mainly from its original description — should be assumed unbred in captivity.
Set plainly: breeding.mode is an egg-scatterer over gravel with no parental care, and the honest expectation for a keeper is that this is not a species anyone breeds at home.
In the aquarium
Nemacheilus kapuasensis is not established in the aquarium trade, and no hobbyist care reports specific to this species were found. What can be said comes from the needs of Nemacheilidae generally: these are fast-water fish that want strong turnover (several times tank volume per hour), high dissolved oxygen, a sand-and-fine-gravel substrate, and a tank kept clean and cool-to-moderate rather than warm and still. A social species of this genus is typically better kept in a small group rather than alone.
Because nemacheilids are near-scaleless, they are notably medication-sensitive — copper-based treatments and full-strength doses of many standard remedies should be avoided or halved, and any newly introduced fish should be watched closely for ich, to which scaleless loaches are often the first in a tank to succumb. Given how little is documented about this particular species, anyone attempting to keep it should treat every husbandry parameter as provisional and err toward the cooler, cleaner, faster-flowing end of what stone loaches generally need.
Conservation
The IUCN Red List assesses Nemacheilus kapuasensis as Least Concern, in an assessment dated 2022. This reflects a species that, while known from a specific Bornean river system, is not currently considered at elevated risk of extinction.
As with many small Bornean stream endemics, the longer-term pressure on this species is likely to be habitat-driven — deforestation, land conversion and associated changes to stream flow and water quality across Kalimantan and Sarawak — rather than any pressure from the aquarium trade, in which the species does not appear to feature.