Taxonomy & naming
Schistura nicholsi was described by Hugh McCormick Smith in 1933, hence the parentheses around his name marking a later change of genus. It belongs to Nemacheilidae, the stone and brook loaches — the largest family of loaches by species count, and one that has been extensively reshuffled as ichthyologists split the old catch-all genera Nemacheilus and Noemacheilus into narrower, better-defined lineages such as Schistura, Triplophysa and Barbatula. The Catalog of Fishes is the standing authority for the valid combination, and it currently places this species in Schistura.
Within the genus, Schistura nicholsi is one of many similar-looking small stream loaches from mainland Southeast Asia, distinguished from its relatives chiefly by fine details of banding, fin-ray counts and other characters documented in the original description and later revisions. As with much of this genus, the taxonomy of Mekong-basin Schistura remains an active area of study, and older literature or trade material may still carry it under a superseded generic combination.
Morphology
This is a small, slender true loach in the classic Nemacheilidae mould: an elongated, subcylindrical body, a flattened belly suited to hugging the streambed, and barbels around a downturned mouth used to feel out food in the substrate. Seriously Fish records a maximum size of around 3 in standard length, making it a modest, unobtrusive fish even at full growth.
Like many of its relatives it carries a small erectile suborbital spine below the eye — a defensive structure shared across the true and spined loaches — and the skin is effectively naked, covered only in tiny, deeply embedded scales. That near-scaleless condition is a recurring theme in this family and matters directly for how the fish should be handled and medicated in the aquarium.
Habitat
Schistura nicholsi is native to the Mekong basin across Laos, Thailand and Cambodia; its type locality is a mountain stream near Pak Jong in Nakhon Ratchasima province, Thailand. It lives in clear, fast-flowing tributary and headwater streams over coarse sand, gravel and rock, typically in forest-shaded reaches where the canopy keeps the water cool and the current stays brisk year-round.
Recorded conditions run from about 68–79 °F with a pH of roughly 6.0 to 7.5 — cool-to-mild, never warm stillwater. This is a fish of moving water: it is intolerant of stagnation, and in the aquarium it needs strong flow, on the order of four to five times the tank volume turned over per hour, plus the high dissolved oxygen that fast water carries with it.
Feeding
Schistura nicholsi is primarily carnivorous, foraging over the stream bottom for small insects, worms, crustaceans and zooplankton — the typical diet of a stone-loach micro-predator working fast, clear water for whatever invertebrate life it can find among the gravel.
In the aquarium it takes live and frozen foods readily, including Daphnia, brine shrimp (Artemia) and bloodworm, and will also accept dried foods once settled. Because it is a small-mouthed bottom forager rather than a surface feeder, sinking foods offered after the more boisterous fish have eaten give it the best chance to get its share.
Mating
Detailed accounts of pairing behaviour in Schistura nicholsi are sparse, consistent with a small stream loach that has rarely been observed spawning under any conditions, wild or captive. No reliable external sexual dimorphism has been documented for this species in the sources available; keepers generally cannot sex the fish by eye with confidence.
What is known comes from the handful of aquarium spawnings on record: ripe fish scatter eggs over a mixed coarse substrate rather than pairing off to guard a nest or cave, in keeping with the broader pattern for Nemacheilidae. Any cues that trigger spawning in the wild — seasonal flow, temperature or photoperiod changes typical of Mekong headwater streams — are not documented in detail for this species.
Breeding
Schistura nicholsi has bred in the aquarium, but only rarely, and the record is thin: it is an egg-scatterer with no parental care, releasing eggs over a coarse, mixed substrate of sand, gravel and stones. Reports note that adults will consume their own eggs and fry, and one documented spawning yielded only seven surviving fry — a reminder of how marginal captive reproduction is for this species.
This fits the honest picture for stone and brook loaches generally: most Nemacheilidae are simply not bred in home aquaria, and Schistura nicholsi is an exception only in that a spawning has actually been recorded, not in that it is in any way reliable or repeatable. A keeper hoping to breed it should expect very low fry numbers and no parental care whatsoever, and should be prepared for eggs and fry to be eaten by the adults themselves.
In the aquarium
Schistura nicholsi is a small, undemanding-looking fish with one demanding requirement: current. It needs strong, well-oxygenated flow — Seriously Fish suggests turning over four to five times the tank volume per hour — over a substrate of sand and gravel with scattered rock and smooth stones to mimic its home streams. Stagnant or sluggish water is poorly tolerated, so this is not a fish for a quiet, low-flow community tank.
Because it is near-scaleless, like other true loaches it is sensitive to copper-based medications and full-strength treatments generally; gentler dosing and careful disease management are the rule. At its modest maximum size of about 3 in it does not need a large footprint, but it does need cool-to-mild, clean, fast-moving, oxygen-rich water and a hardscape that lets it forage and hide among gravel and stone — a scaled-down slice of a Mekong headwater stream rather than a warm, still blackwater setup.
Conservation
Schistura nicholsi is assessed by the IUCN as Least Concern. Its range across the Mekong basin in Laos, Thailand and Cambodia is wide enough that local pressures such as logging and agricultural river modification have not been judged sufficient to push the species into a higher threat category.
As with many small-range Mekong-basin stream loaches, the wider concern is less about this particular species' current status than about the pace of land-use change across Southeast Asian headwaters generally; a wide distribution offers some buffer, but forest-stream specialists like this one remain dependent on the continued presence of clear, fast, shaded water rather than any single protected site.