Taxonomy & naming
Triplophysa arnoldii was described by Alexander Prokofiev in 2006. It sits in the genus Triplophysa, part of the family Nemacheilidae — the stone or brook loaches, by far the largest loach family and one that has absorbed many species formerly parked in the old catch-all genera Nemacheilus and Noemacheilus as those groups were repeatedly split apart. Triplophysa itself is enormous, with well over a hundred described species spread from the Himalaya and Tibetan plateau across Central Asia, and the Catalog of Fishes is the standing authority for which combination is currently valid.
The species epithet honours Lev Vladimirovich Arnoldi (1903–1980), a Russian entomologist and hydrobiologist. Beyond the original description, published literature on this species is sparse, which is typical for a narrow-range Central Asian Triplophysa — many were described from a handful of specimens collected on a single expedition and have not been revisited since.
Morphology
Reported data give a maximum length of about 3 in standard length, making Triplophysa arnoldii a small member of the genus. FishBase records the fin and vertebral counts from the original description: three dorsal spines and seven dorsal soft rays, two to three anal spines and five to six anal soft rays, and 39–42 vertebrae — the kind of meristic detail that distinguishes closely related Triplophysa species from one another rather than anything visually dramatic.
Like other Nemacheilidae it is a slender, elongate, bottom-hugging fish with a flattened underside suited to holding position on stones in current, rather than the deep-bodied, boldly banded build of the botiid loaches. Data on colour pattern and the finer points of its morphology beyond the counts above are sparse, so specifics are best left unstated rather than guessed.
Habitat
Triplophysa arnoldii is endemic to Western Mongolia, a landlocked, high-elevation region of Central Asia. Nothing further about its exact type locality or the particular river system has surfaced in the sources checked, and its precise habitat requirements are not documented in detail — a common situation for narrow-range Central Asian stone loaches known from limited collecting.
What can be said follows from its family and region: Nemacheilidae in this part of Central Asia are characteristically fish of cool, clear, fast-flowing, well-oxygenated streams running over stone and gravel substrate, and closely related Triplophysa species from similar latitudes and elevations are documented living in cool water around 61–68 °F. It is reasonable to expect this species occupies similar fast boulder-and-gravel stream habitat, though this should be read as an inference from its relatives rather than a confirmed account of this species itself.
Feeding
No species-specific diet information was located for Triplophysa arnoldii. Stone loaches in Nemacheilidae are typically bottom-foraging micro-predators, picking small benthic invertebrates and insect larvae from among stones and gravel in the current, and there is no reason to expect this species departs from that pattern.
In the absence of a documented aquarium diet, the safe assumption for a fish of this build and habitat is that it would take small live and frozen foods and sinking preparations off the bottom, in the manner of its better-known Triplophysa and Schistura relatives, rather than surface flake.
Mating
No information on courtship, sexing, or spawning behaviour for Triplophysa arnoldii was found in the sources checked. This is unrecorded rather than unusual — the great majority of Nemacheilidae species, including most Triplophysa, have never had their reproductive behaviour observed and described, whether in the wild or in captivity, and this species appears to be one of them.
Breeding
There is no documented aquarium or captive breeding record for Triplophysa arnoldii, and the species does not appear to have been kept in the hobby at all. Based on the general pattern for stone and brook loaches, the expected mode is an egg-scatterer that releases eggs over gravel or among stones in fast-flowing water with no parental care — but this is inferred from the family rather than observed in this species, and it should be treated as a reasonable expectation rather than a documented fact. In practice, this is a fish that is essentially never bred in captivity, and data on its reproduction remains sparse.
In the aquarium
Triplophysa arnoldii is not an aquarium fish in any practical sense — it has no documented presence in the hobby, and there is no established care profile to draw on. What follows is inference from its family, region and relatives, offered cautiously rather than as tested guidance.
As a small (around 3 in), near-scaleless Nemacheilidae stone loach from a cool Central Asian stream system, it would be expected to need cool, clear, fast-flowing, well-oxygenated water over a stone-and-gravel substrate with a mature biofilm surface to graze and forage on — not the warm, still conditions suited to many tropical aquarium fish. Its thin, largely scaleless skin, typical of loaches generally, would make it sensitive to copper and to full-strength medication doses, meaning any treatment should be approached cautiously and at reduced strength. Given the total absence of husbandry data, this is not a species to acquire speculatively; anyone encountering it would be working almost entirely without a map.
Conservation
Triplophysa arnoldii has not been assessed on the IUCN Red List and is classified here as Not Evaluated. No population trend, threat assessment, or trade data were found in the sources checked.
As a narrow-range endemic known only from Western Mongolia, it fits the broader pattern seen across Triplophysa and Nemacheilidae generally: many species are restricted to a single stream system or small region, are known from limited historical collecting, and have never been formally assessed for conservation status. Without a Red List evaluation or documented survey data, its true conservation situation is unknown rather than presumed safe.