Taxonomy & naming
Schistura geisleri was described by Maurice Kottelat in 1990. Records show it was originally treated within the broader Nemacheilus group before being placed in Schistura as that genus was clarified — a pattern repeated across many of the old "Nemacheilus"/"Noemacheilus" stone loaches, which molecular and morphological work has since split among Schistura, Triplophysa, Barbatula and other genera. The Catalog of Fishes, the authority for valid names, retains it in Schistura within the family Nemacheilidae, the stone and brook loaches — by far the largest family in the loach superfamily Cobitoidea.
Schistura is a huge genus of small, superficially similar Asian stream loaches, and species-level identification often rests on fine details of colour pattern, fin-ray counts and the shape of the suborbital spine rather than anything visible at a glance. As with much of this genus, S. geisleri is known mainly from its type region rather than from a broad, well-studied range.
Morphology
This is a small, slender, cylindrical loach reaching only around 1.5 in in standard length — tiny even by stone-loach standards. Like other nemacheilids it is a true loach: a bottom-hugging body built for holding position against current, small eyes set high on the head, and barbels around the mouth for feeling out food on the substrate. The skin is effectively naked, with the tiny embedded scales typical of the family offering little protection and making the fish, like its relatives, sensitive to poor water quality and heavy-handed medication.
A notable identification feature in males is a distinctive hammer-shaped suborbital flap, a modified structure below the eye that stands in for the erectile suborbital spine seen across many stone loaches and their botiid cousins; this flap is the clearest external difference between the sexes. When startled, S. geisleri buries itself in fine substrate, a typical stone-loach escape response.
Habitat
Schistura geisleri is native to Thailand, recorded from the Chao Phraya basin and from rivers of peninsular Thailand including the Tapi system. It lives in shallow streams with decent current and high dissolved oxygen, over beds of smooth pebbles and sand, typically in clear, well-lit water rather than shaded blackwater.
This is a running-water fish through and through: the combination of shallow depth, brisk flow and coarse mineral substrate is what keeps the water oxygen-rich enough for a scaleless, current-adapted loach like this one. Reported temperatures span roughly 64–79 °F across sources, a cool-to-moderate range consistent with a small, fast, well-aerated stream rather than a warm, still pond.
Feeding
Schistura geisleri is a carnivorous micro-predator, foraging over the stream bed for small insects, worms and crustaceans. In the aquarium it takes frozen and live foods readily — bloodworm, brine shrimp (Artemia) and Daphnia are all reported to be accepted — alongside commercial sinking pellets aimed at the substrate rather than the surface.
Because it is a small-mouthed bottom forager rather than a generalist grazer, flake food that sits in the water column is easy to miss; feeding is more reliable when sinking foods are offered directly at loach level, ideally after other tankmates have had their fill.
Mating
The clearest sexual signal in this species is the male's hammer-shaped suborbital flap, which is absent or much less developed in females — a useful field mark for anyone trying to assemble a breeding group. Beyond that, detailed accounts of courtship behaviour are not well documented for this species; data is sparse, as is typical for many small Schistura known chiefly from their type streams.
One captive spawning account describes success using a ratio of two males to four gravid females in a dedicated tank, suggesting that, as in many stone loaches, a female-skewed group and a stimulus tank separate from general maintenance may improve the odds.
Breeding
Schistura geisleri is an egg-scatterer with no parental care, consistent with the breeding pattern across Nemacheilidae: ripe fish release eggs over gravel in fast-flowing water and take no further part in rearing them. It is rarely bred in the home aquarium, and accounts of successful captive spawning are limited — one source describes a deliberate attempt using separate spawning tanks with coarse gravel and a female-skewed group, while another source reports no documented captive breeding at all, which points to genuinely inconsistent or difficult reproduction rather than an established hobby method.
As with most stone and brook loaches, a keeper should not expect to breed this species reliably; the honest picture is occasional, hard-won success rather than a repeatable protocol.
In the aquarium
Schistura geisleri suits a specialist stream-style setup rather than a generic community tank. Seriously Fish recommends a minimum footprint of around 60×12 in, modest for the fish's tiny adult size but reflecting its need for open swimming and foraging space along the substrate. It is peaceful and best kept in a group of four or more, which encourages natural, confident behaviour instead of the skittishness a lone or paired individual tends to show.
Good water quality matters more than tank size: this is a fast-water fish that wants high dissolved oxygen, strong filter turnover (on the order of four to five times the tank volume per hour has been suggested), and regular substantial water changes — 30–50% weekly is a reasonable target. A smooth pebble-and-sand substrate lets the fish bury itself when startled, and because it is essentially scaleless, medications should be dosed cautiously; full-strength treatments and copper-based products are best avoided or used at reduced dose. Being rare in the trade, most specimens available to hobbyists are likely wild-collected rather than farmed.
Conservation
Schistura geisleri is assessed as Least Concern on the IUCN Red List. It is not a species under known heavy collection or trade pressure — it appears infrequently in the aquarium trade — and no major threats are documented in the sources available here.
As with many small stream endemics restricted to particular river systems, habitat degradation, pollution and changes to water flow within the Chao Phraya and peninsular Thai river basins are the general long-term risks to watch for such species, even where the current assessment is favourable.