Taxonomy & naming
Erromyzon compactus was described by Maurice Kottelat in 2004. Kottelat has been the dominant figure in Indochinese freshwater ichthyology for several decades, and the description of this species fits within his wide-ranging work on the loaches and cyprinids of Yunnan and northern Indochina. The genus Erromyzon belongs to the family Balitoridae — the torrent loaches or hillstream loaches — a group distributed across the fast-flowing upland streams of South and Southeast Asia. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid name; the genus and species combination Erromyzon compactus stands as described.
Balitoridae as a whole was substantially reshuffled in the early 2000s and again following molecular work through the 2010s; what was once a single sprawling family of 'river loaches' has been partitioned into several families and many rearranged genera. Erromyzon itself is a small genus of Indochinese hillstream loaches, and E. compactus is one of the more recently formalised members. No synonym is currently listed for this species, and no common name has been formally established, though the trade sometimes sells it under the vernacular 'compact hillstream loach.'
Morphology
Like all balitorid torrent loaches, Erromyzon compactus is strongly dorsoventrally flattened — the body is nearly as wide as it is tall — with paired pectoral and pelvic fins spread horizontally to form a broad, low-slung hydrodynamic platform that presses against the substrate in fast current. The underside is flattened and in combination with a slightly modified oral disc allows the fish to grip rock surfaces securely without being swept downstream. This body plan is an evolutionary convergence with suckermouth catfishes: the fish is not related to loricariids but has arrived at a similar solution to the same problem.
Maximum standard length is around 1.5–2 in, placing it firmly among the smaller hillstream loaches. The body is moderately compact relative to other Erromyzon species, which is reflected in the species epithet. The skin is effectively scaleless or very finely scaled — a characteristic shared with most loaches — making the fish sensitive to copper-based medications and other chemical treatments. A suborbital erectile spine, characteristic of loaches more broadly, is present. Colour is typically a mottled brown or brownish-grey with darker patterning on the back and sides, providing camouflage against rocky substrates in clear water.
Habitat
Erromyzon compactus is endemic to hill streams draining southern Yunnan province in China and northern Vietnam — a region of rugged karst topography and montane river systems that funnel clear, fast water down steep gradients over rock and gravel substrate. These streams run cool, well-oxygenated and swift, with temperatures in the range of 61–72 °F and pH typically 6–7, reflecting soft to moderately hard mountain water. The streambed is dominated by exposed rock, stones and coarse gravel, with sparse silt and abundant epilithic algae and biofilm where light reaches the surface.
This is a rheophilic species — it is an obligate resident of fast-flowing, high-oxygen environments and is not found in still or slow water. The current velocity and dissolved oxygen content of its native streams cannot be replicated with ordinary aquarium filtration; it requires purpose-built high-flow setups that approximate the torrent conditions it has evolved to inhabit.
Feeding
Erromyzon compactus is an aufwuchs grazer. In the wild it scrapes algae, diatoms, microinvertebrates and organic detritus from the surface of submerged rock using its subterminal, semi-modified mouth — a form of feeding common to hillstream loaches and analogous to the rasping of loricariid catfishes. The diet is predominantly plant- and biofilm-based in the field, supplemented by the small invertebrates embedded in biofilm communities.
In the aquarium, feeding this species is the single most common point of failure. A bare or newly-set-up tank offers nothing to graze, and the fish slowly starves even in chemically acceptable water. A successful setup requires a mature rockscape with a well-established coating of algae and biofilm — typically meaning several months of growth under bright lighting before the fish is introduced. Supplement this living larder with blanched greens (courgette, cucumber, spinach), and occasional small frozen foods such as daphnia or finely chopped bloodworm. It will not thrive on flake food sinking to the bottom and should not be expected to clean a tank of nuisance algae in the manner of a common plecostomus.
Mating
Virtually nothing is documented about the reproductive biology or sexual dimorphism of Erromyzon compactus from wild observations or aquarium records. In keeping with the broader hillstream loach pattern, females are likely distinguishable from males only by a fuller abdomen when gravid. The erectile suborbital spine is not reliably dimorphic in balitorids.
In their native streams, hillstream loaches in general are presumed to scatter eggs among gravel and stones in moving water, with no parental investment beyond the act of spawning. Seasonal and environmental cues — water temperature changes with altitude and season, current velocity, substrate condition — are assumed to govern reproductive timing, but specific data for this species are lacking.
Breeding
Erromyzon compactus has not been bred in home aquaria to any documented degree. It is an egg-scattering loach with no parental care — the reproductive strategy shared across the balitorid hillstream group. A small number of hillstream loach species, most notably Sewellia lineolata, have been spawned in mature, strongly-flowing river tanks, with eggs deposited among gravel and stones and left unattended. Whether E. compactus would follow the same pattern given optimal conditions is unknown; the species is sufficiently rare in the hobby that no verified spawning account exists.
As with most loaches, the honest position is that breeding in home aquaria is rarely if ever achieved, and a keeper should not expect it. For this species, even maintaining adults in good long-term condition is a significant challenge; reproduction is not a realistic goal for most keepers. The aquarium trade supply is entirely wild-collected, from the hill streams of Yunnan and northern Vietnam.
In the aquarium
Erromyzon compactus is a specialist's fish. Its core requirements — cool water, extremely strong flow and high dissolved oxygen, and a mature biofilm-covered rockscape — are difficult to achieve and maintain, and a setup that meets all three is very different from a general tropical community tank. Water temperature should be kept within the 61–72 °F range; temperatures consistently above 75 °F stress or kill the fish, which rules out the warm conditions most tropical fish require. A purpose-built hillstream tank with a powerhead or circulation pump producing a strong, turbulent flow — typically 1,0.5 US gal per hour or more in a modest-sized tank — and abundant smooth or rough stone surfaces is the foundation.
The tank must be mature before the fish is introduced, with rocks colonised by algae, diatoms and biofilm. A newly-set-up tank is a starvation environment regardless of other conditions. Given its near-scaleless skin, E. compactus is sensitive to copper and should receive only half-doses of any medication, with particular caution around copper-based ich and parasite treatments; yet like many loaches it may be the first fish in a tank to develop ich, so close observation is essential. This species appears to be tolerant of or even social in the presence of conspecifics and other hillstream loaches in the wild; keeping a small group is reasonable where the tank size and biofilm supply allow. It should not be housed with large, boisterous, or warm-water tankmates.
Conservation
Erromyzon compactus has not been formally assessed by the IUCN Red List and so carries no official conservation category; it is recorded here as Not Evaluated. Many small-range Balitoridae from the hill streams of Yunnan and northern Indochina fall into this gap — they are too recently described or too poorly known to have attracted a full assessment, despite often being geographically restricted endemics in catchments under significant pressure.
The hill streams of southern Yunnan and northern Vietnam face a range of threats common to montane river systems in the region: dam construction and altered hydrology, siltation from agriculture and land clearance, water extraction, and localised pollution. A species with a small natural range and dependence on cool, fast, clear, high-oxygen water is inherently vulnerable to any degradation of flow or water quality. The aquarium trade draws on wild-collected specimens, and while the volume traded is almost certainly low given the species' limited availability, wild collection from a restricted range adds a further pressure. No population data are available.