Taxonomy & naming
Erromyzon sinensis was described by Chinese ichthyologist Chen in 1980, originally placed in the genus Protomyzon, a name that has since been applied differently across the literature. In 2004, the influential taxonomic revision by Maurice Kottelat established the genus Erromyzon to accommodate this fish and its close relatives, and the Catalog of Fishes — the international authority for valid names — now recognizes the combination Erromyzon sinensis (Chen, 1980), with parentheses indicating the original genus has changed. It is the type species of the genus Erromyzon.
Erromyzon belongs to the family Gastromyzontidae (classified as Balitoridae in older literature and still in some databases including FishBase), a group of dorsoventrally flattened hillstream loaches from fast-flowing rivers of East and Southeast Asia. The genus Erromyzon is small and contains only a handful of described species, all from the drainages of southern China. The species is sometimes listed under older names or as Protomyzon sinensis in aquarium trade literature; the current valid combination is Erromyzon sinensis.
Morphology
Like other gastromyzontid hillstream loaches, Erromyzon sinensis is radically flattened from belly to back — a body plan evolved not for swimming through open water but for clinging to current-swept stone. The pectoral and pelvic fins are spread horizontally and, together with the flattened ventral surface, form an effective adhesive platform that allows the fish to maintain position on vertical rock faces and bare boulders in torrential flow. Adults reach approximately 2 in standard length (around 2 in), making this a compact species.
The head is broad and depressed, with a subterminal or inferior mouth adapted for scraping biofilm from hard substrates. The lips are fleshy and papillate. The skin is near-scaleless in texture, as in many hillstream loaches, which has important implications for medication sensitivity. A suborbital spine — the erectile defensive spine characteristic of several loach lineages — is present. Body colouration is mottled brown and cream, providing effective camouflage against the mottled rock and gravel of fast-stream beds.
Habitat
Erromyzon sinensis is native to the Xi Jiang (West River) system of Guangxi Autonomous Region in southern China, including the Li Jiang and Gui Jiang drainages. This is subtropical hill-stream terrain: clean, fast-flowing, highly oxygenated water running over boulders and smooth rock, with significant current at all times. Water temperatures in the natural habitat range from roughly 64–75 °F, and the fish originates from a region where winters can be cool — it does not inhabit the warm, slow, lowland tropical rivers that many aquarium fish call home.
The substrate is dominated by large smooth stones, bedrock faces and coarse gravel, all coated in the biofilm and algal mat that this species depends on for food. The water is characteristically clear, well-oxygenated and moderately soft to moderately hard (pH 6.0–7.5 in some accounts, 7.0–8.0 in others), indicating a somewhat variable water chemistry across its range. Dissolved oxygen levels are high year-round due to the nature of the torrent habitat. This is not a still-water or lowland species.
Feeding
Erromyzon sinensis is an aufwuchs and biofilm grazer, rasping the thin layer of algae, diatoms, bacteria and organic detritus from the surface of submerged rock. In its native streams this food source is essentially unlimited on every exposed boulder face, continually renewed by the high-flow, well-lit conditions. The fish's inferior, papillate mouth and muscular lips are precisely adapted for this mode of feeding.
In the aquarium, the first feeding requirement is a biologically mature tank with well-established algae and biofilm growth on rocks and glass. A fish placed in a newly-set-up bare tank will slowly starve, because the surface food it needs has not had time to develop — this is the number-one dietary failure with this species. Supplementary foods accepted include algae wafers, spirulina-based sinking products, blanched green vegetables (spinach, kale, courgette), and small frozen foods such as bloodworm, mysis and artemia nauplii. Strong overhead lighting (8+ hours daily) should be maintained to encourage the growth of natural algae on rocks and glass, which remains the fish's most important dietary baseline.
Mating
Little is documented about the mating behaviour of Erromyzon sinensis specifically. In the closely related and similarly shaped hillstream loaches, males and females are often subtly distinguishable only when females are visibly gravid: ripe females become noticeably plumper in the abdominal region, while males may be slightly smaller and more slender. Any courtship behaviour in the wild presumably involves positioning over or among rocks in high-flow conditions, as has been inferred from the few captive spawning accounts of near relatives.
A closely related species, described informally as Erromyzon sp. 'ER02', has been observed adopting a T-shaped spawning position on rock faces in captivity, suggesting that the genus's breeding behaviour involves direct spawning contact between male and female against a firm substrate in flow. Whether Erromyzon sinensis itself follows exactly the same pattern is not confirmed, but the structural similarity between species in the genus makes a broadly similar mating system plausible.
Breeding
Captive breeding of Erromyzon sinensis is almost entirely undocumented; no verified reports of spawning in a home aquarium are on record for this species specifically. Like most hillstream loaches, it would be expected to be an egg-scatterer in fast water, depositing eggs among stones or on rock surfaces with no parental care — the eggs and larvae develop in the flow without adult attendance.
For most genera of hillstream loaches, captive spawning is rare and demanding, requiring a mature, strongly-flowing, well-oxygenated river tank with established biofilm and appropriate cool temperatures. The closely related Erromyzon sp. 'ER02' has reportedly spawned in captivity under such conditions, which is the nearest data point available. Any keeper hoping to attempt breeding Erromyzon sinensis should concentrate first on replicating the natural fast-flow, cool, mature environment; spawning, if it occurs at all, will be incidental to getting those fundamentals right. The trade supply, such as it is, appears to rely on wild-collected fish rather than captive production.
In the aquarium
Erromyzon sinensis is an extremely rare fish in the trade, demanding in its requirements, and not a beginner's species. The most critical requirements are: (1) strong, continuous water flow — at least 10–15 times the tank volume per hour via powerheads or a strong sump return aimed across smooth rock surfaces; (2) high dissolved oxygen at all times, achieved through surface agitation and the flow itself; (3) cool water in the 64–75 °F range — this fish should NOT be kept at tropical aquarium temperatures of 79–82 °F, which will stress or kill it over time; and (4) a biologically mature tank with an established biofilm and algal coating on rocks and smooth surfaces, without which the fish will slowly starve.
Smooth river stones, slate, and unglazed ceramic structures are the preferred decor; avoid sharp-edged rock that could injure the soft belly and fins. Sand mixed with smooth pebbles makes an appropriate substrate to a depth of at least 2 in. A secure, gap-free lid is essential — this species can and will climb glass and escape. Tankmates should be chosen carefully: other hillstream species with identical water requirements can coexist, but typical tropical community fish with warm, still-water needs are incompatible. Loaches Online recommends keeping this fish in groups; the Aquarium Co-op care context for hillstream loaches generally supports a group of four or more. Because the skin is near-scaleless, treat with caution if medication is ever required — use half-doses of any copper-based or organophosphate treatment, and remove carbon during treatment. Watch for ich, to which hillstream loaches in suboptimal conditions are vulnerable.
Conservation
Erromyzon sinensis is assessed as Data Deficient (DD) by the IUCN Red List, reflecting the limited information available on its population size, range and threats. The species is known from a restricted area of Guangxi Province in southern China, within the Xi Jiang watershed, and appears to be genuinely uncommon even within that range — it is described as extremely rare in the aquarium trade, which is itself an indirect indicator of how infrequently collectors encounter it.
The fast, clear hill-stream habitats it depends on are vulnerable to the impacts of agricultural runoff, dam construction, river channelisation and water extraction — pressures well documented across southern China's river systems. Its apparently small natural range and specialised habitat requirements (torrent-zone, biofilm-rich rock substrates) mean that even localised degradation could have outsized effects on a population. The trade is not a significant pressure given how rarely the fish appears commercially, but wild-collected individuals are unlikely to be sourced sustainably given the absence of any captive production. More field survey work is needed before the species' true conservation status can be properly evaluated.