Taxonomy & naming
Erethistes pusillus was described by the German naturalists Johannes Müller and Franz Hermann Troschel in 1849 in their Horae Ichthyologicae, from Assam, India; the unique holotype is in Berlin (ZMB 3102). The species was the original — and, by monotypy, the type — species of the new genus Erethistes, which Müller & Troschel initially proposed as a subgenus within the then-sprawling catch-all Bagrus. That historical subgenus arrangement is the only sense in which this fish was ever a 'Bagrus', and it explains why the name occasionally surfaces coupled with Bagrus in old indexes; Eschmeyer's Catalog of Fishes records the original combination as Erethistes pusillus, and the authority is written without parentheses because the species has never left its original genus.
Family placement has moved around the fish. The small South Asian hill-stream genera around Erethistes and Hara were for a time separated as their own family, Erethistidae; the classification followed by Eschmeyer's Catalog of Fishes returns them to the Sisoridae (subfamily Sisorinae). The IUCN assessment further notes that Hara may not be morphologically separable from Erethistes — the two differ mainly in the direction of the serrations on the front edge of the pectoral spine — in which case Erethistes, as the senior name, would absorb it.
The current status per Eschmeyer's Catalog of Fishes is: valid as Erethistes pusillus Müller & Troschel 1849, family Sisoridae.
Morphology
This is a genuinely tiny catfish: maximum recorded size is only about 1.7 in standard length, among the smallest catfishes an aquarist is likely to encounter. Like other sisorids it has a flattened head and body adapted for clinging to rocks and holding position in current, short barbels, and the naked, scaleless skin typical of catfishes generally.
As with other scaleless catfish, that bare skin makes it more sensitive than scaled fish to medications — copper-based treatments and many general remedies should be dosed conservatively, ideally at reduced strength, exactly as with loaches, puffers, and other naked-skinned groups. Its small pectoral and dorsal fin spines are far less of a hazard than the stout, locking spines of larger catfish relatives, but netting should still be done gently to avoid tangling delicate fins.
Habitat
Erethistes pusillus is native to South Asia: the Ganges and Brahmaputra drainages of Nepal, India, and Bangladesh, with records from Myanmar. The IUCN assessment describes its habitat as the slower stretches of the upper reaches of rivers, over predominantly sandy bottoms with considerable vegetation — overhanging or growing in the water — and FishBase likewise records it mainly from hill streams and from muddy, heavily vegetated parts of rivers. It is, in other words, a fish of quiet, structured margins in upland drainages rather than of open torrent.
Independently verified pH, hardness, and temperature figures for this species are not available, and rather than guess at numbers, the safest guidance is habitat-based: clean, well-oxygenated water with gentle-to-moderate movement over sand, with plant or root cover, matching the vegetated stream margins it occupies in nature.
Feeding
In the aquarium, Erethistes pusillus takes live and frozen foods readily — bloodworm, white worms, daphnia, and brine shrimp are all reported as accepted foods. Being so small, it needs correspondingly small prey items; it is not a piscivore and poses no threat to normal community tankmates.
It is a shy, nocturnal bottom-dweller, so food is best offered after lights-dim or directly near cover where it feels secure enough to forage, rather than expecting it to compete in the open with more boisterous tankmates.
Mating
No spawning behaviour has been documented for this species in the sources available. Sisorid hill-stream catfishes as a group are not known for elaborate courtship displays reported in the aquarium literature, and specific mating behaviour for Erethistes pusillus is simply not recorded in the available sources. Data sparse — this account does not speculate beyond what is documented.
Breeding
Erethistes pusillus is not reported to be bred in home aquaria. The species is essentially unknown from captive spawning accounts; hobbyist sources describe it only as an aquarium subject to be kept and observed, not propagated. The IUCN assessment records that it is collected and exported as an ornamental fish, so trade supply is understood to be wild-collected rather than farmed.
In the aquarium
Because of its very small adult size — about 1.7 in SL — Erethistes pusillus needs only a modest tank. It is described as peaceful and shy, best suited to a small tank with calm tankmates such as small tetras, rasboras, or Corydoras, subdued lighting, and plenty of hiding places (smooth stones, driftwood, leaf litter, plant thickets) to replicate the vegetated stream margins it comes from. Clean, well-oxygenated water with gentle-to-moderate movement suits it; it is a fish of quiet, structured margins rather than torrents, so extreme flow is unnecessary.
Because it is scaleless, treat it cautiously with medications — copper and many standard treatments should be used at reduced strength. It is nocturnal and best appreciated with dim lighting or a torch after dark; daytime hides are essential. PlanetCatfish reports a lifespan of more than five years with proper care. Given its rarity in the trade and small size, it is a fish for dedicated small-species keepers rather than a general community staple.
Conservation
Erethistes pusillus is assessed as Least Concern on the IUCN Red List (Ng 2010; assessed 12 October 2009). The assessment notes that although the exact distribution is not fully mapped, the species appears to be relatively common wherever it occurs, and that collection for the ornamental fish trade is currently at a level not projected to adversely affect the global population — while recommending future monitoring of harvest levels and identification of potential habitat threats.
Its range spans the Ganges–Brahmaputra catchment, where damming, pollution, and stream degradation are broad pressures on aquatic life; species-specific threat data are lacking, the population trend is recorded as unknown, and the Red List annotation flags the assessment as needing an update.