Taxonomy & naming
Acanthopsoides gracilentus was described by Hugh M. Smith in 1945, placing it in the genus Acanthopsoides within the family Cobitidae, the true or spined loaches. The Catalog of Fishes, the standing authority for valid fish names, continues to recognise the species under this combination, and no synonymy or generic transfer is noted here — the genus name is used as given.
Cobitidae is one of several loach families in the superfamily Cobitoidea, distinguished chiefly by the erectile, two-pronged spine that sits in a pocket beneath each eye. Acanthopsoides is a small genus of slender, sand-associated loaches restricted to Indochina, and gracilentus — Latin for "slender" or "thin" — describes the fish's elongated, low-slung body shape well.
Morphology
This is a slim, elongated loach with a pointed snout, small eyes set high on the head, and the erectile suborbital spine typical of Cobitidae, tucked in a groove below each eye and flicked out when the fish is handled or threatened. The body is covered in tiny, deeply embedded scales that leave the skin functionally naked — a trait shared across true loaches and one that matters for how the fish tolerates medication.
FishBase records a maximum size of about 2.5 in standard length, making Acanthopsoides gracilentus one of the smaller members of its family. Colouration in cache sources is not well documented; like many sand-dwelling loaches it is likely to show a pale, sandy base tone with fine mottling or barring that breaks up its outline against a sand or fine-gravel bed.
Habitat
The species is native to the Mekong and Chao Phraya river basins, spanning Thailand, Laos and Cambodia. It occurs in medium to large rivers over sandy bottoms, in water ranging from slow to fast-flowing, often where sand gives way to gravel and pebbles. This is a coolwater loach rather than a warm tropical one: Seriously Fish records it living at 68–75 °F in the wild, in soft, acidic water.
It is a burrowing, sand-sifting specialist, spending much of its time buried in or moving through the substrate rather than swimming in open water. Flowing river channels with a clean sand or fine-gravel bed are its natural setting, and that substrate is not incidental — the fish depends on it to feed and to hide.
Feeding
Acanthopsoides gracilentus is a sand-sifting benthic invertebrate feeder. In the wild it buries itself in the substrate and sifts sand through its gills, straining out small worms, insect larvae, tiny crustaceans and other invertebrate prey along with detritus. This feeding style is closely tied to its slender, burrowing body shape.
In the aquarium it has been kept as a carnivore on sinking pellets and live or frozen foods such as bloodworm and small worms. Because natural feeding depends on sifting a soft substrate, a bare-bottom or coarse-gravel tank denies the fish its normal foraging behaviour; fine sand is close to a requirement rather than decor.
Mating
No dimorphism or courtship behaviour is documented for this species in the available sources, and nothing here should be assumed by analogy with better-known loaches. Given how rarely the fish is kept or bred, even basic sexing criteria are effectively undescribed.
What is known points to a difficult species to spawn deliberately: Seriously Fish reports only one documented captive breeding, achieved over a mixed sand and fine-gravel substrate, with no detail on courtship or spawning triggers preserved. This should be read as an exception rather than a template.
Breeding
Acanthopsoides gracilentus is a spined-loach (Cobitidae) egg-scatterer, and the honest picture is that it is essentially never bred in the aquarium. Unlike the temperate Cobitidae — European spined loaches (Cobitis, Sabanejewia) and the dojo or weather loach (Misgurnus) — which have been pond- or tank-spawned by hobbyists and breeders, the tropical and coolwater Cobitidae of Southeast Asia, including this genus, generally have not. A single documented captive spawning is recorded for this species, with eggs scattered with no evidence of parental care, but it stands as a rarity rather than a repeatable method.
Parental care is absent in this group: eggs are scattered and abandoned, with no cave-guarding, no mouthbrooding and no post-spawn attention from the adults. Anyone acquiring this species should expect to keep it as a display and community fish, not to breed it.
In the aquarium
This is a small, coolwater, sand-dependent loach and should be set up accordingly. Aquarium reports suggest a workable range of roughly 72–81 °F with soft, slightly acidic to neutral water (around pH 6.5–7.0), which sits toward the cool end for a loach and should not be pushed into full tropical warmth. A tank of at least 20 US gal with moderate flow and a soft sand substrate is the baseline; the fish needs to be able to bury and sift, so fine sand rather than gravel is important.
Being a scaleless true loach, it is sensitive to copper and to full-strength medications, and should be dosed cautiously if treatment is needed. It is described as gregarious and best kept in a group of six or more. Overall it is regarded as a challenging aquarium subject and is rare in the hobby, with a reported lifespan of three to five years — a fish for keepers prepared to replicate a sandy, flowing river-bottom niche rather than a beginner community loach.
Conservation
Acanthopsoides gracilentus has not been evaluated by the IUCN Red List, and no assessment could be located in cache sources, so its conservation status is recorded here as Not Evaluated rather than guessed. This is a common situation for smaller, non-commercial river-bottom fishes of the Mekong and Chao Phraya systems, which have received far less assessment attention than headline aquarium species.
The Mekong and Chao Phraya basins face well-documented pressures from damming, sand mining and water development, all of which can affect river-bottom sand and gravel habitat of exactly the kind this species depends on, but no source in hand ties a specific population trend or threat level to this fish. It is rare in the aquarium trade, and there is no indication that hobby collection is a meaningful pressure on it.