Taxonomy & naming
Gyrinocheilus aymonieri was described by the French naturalist Gabriel Tirant in 1883, originally as Psilorhynchus aymonieri, based on specimens collected from the waters of what is now Vietnam and Cambodia. The parentheses around Tirant's name signal that the species has since been reassigned to its current genus. The genus Gyrinocheilus was established by Vaillant in 1902, and the species is now firmly placed there; the Catalog of Fishes (Eschmeyer, CAS) recognises it as Gyrinocheilus aymonieri, the type and most widespread species of the genus.
The family Gyrinocheilidae is monogeneric — Gyrinocheilus is its only genus — and the family sits within the order Cypriniformes, making it a distant relative of carp, barbs and true loaches rather than a loach itself in the strict sense. The genus contains three accepted species: G. aymonieri, the widespread common form; G. pennocki from the Thai-Malay peninsula; and G. pustulosus from the Mekong, distinguished partly by scale and tubercle counts. Fish of this genus have long been sold under the erroneous name "Chinese algae eater" — the species is not native to China — or "Siamese algae eater," a trade name that at least places it closer to its actual range in mainland Southeast Asia.
Morphology
Gyrinocheilus aymonieri is an elongated, moderately stout fish with a slightly flattened underside and a rounded snout. The most distinctive feature is the mouth: a ventrally placed sucker disc fringed with fleshy lobes, modified for rasping algae and biofilm from hard substrates in fast current. Crucially, the gill chamber is equipped with separate inhalant and exhalant openings, so the fish can ventilate while its mouth is pressed against a surface — a unique anatomical solution to the problem of breathing while feeding in flow. The body is covered in small scales, the fin membranes are thin, and juveniles show a brown lateral stripe along the flank that tends to break into spots with age.
Maximum reported length is around 11 in standard length, though most trade fish are seen well under 4 in when purchased. The difference between a 2 in juvenile in a shop tank and the full-grown adult it will become is one of the most consistently underestimated realities in the hobby. Adults are robustly built, capable of defending territory forcefully, and large enough to dominate most community aquarium inhabitants.
Habitat
This species is native to the Mekong and Chao Phraya river drainages of mainland Southeast Asia — Thailand, Laos, Cambodia and Vietnam are all part of its native range. It inhabits fast-flowing, well-oxygenated rivers and large streams with rocky or gravelly substrates, where it clings to stones and submerged surfaces in the current to feed. The water is warm, typically 72–79 °F, close to neutral in pH (6.5–7.5), and high in dissolved oxygen from the turbulent flow.
This is not a lentic or slow-water fish. In the wild it is found on current-swept rock faces and rapids where few other species can hold station, and its entire anatomy — the sucker mouth, the dual gill openings, the strong, streamlined body — is shaped by that niche. In the aquarium it tolerates lower current but does best with good water movement and high oxygenation, and it is noticeably less well-suited to the warm still water of a typical community tank than its ubiquitous presence in that setting might suggest.
Feeding
The feeding biology of Gyrinocheilus aymonieri is where the "algae eater" label does the most damage. Juveniles are genuine biofilm grazers: they rasp algae, diatoms, aufwuchs and bacterial mats from hard surfaces with their sucker mouth, and in a tank with a green film of algae, a small Gyrinocheilus will visibly clean the glass. That juvenile behaviour is what sells the fish.
As the fish grows, the picture changes substantially. Adults become increasingly omnivorous and protein-hungry, demanding sinking pellets, blanched vegetables and meatier foods in addition to any algae available. More critically, the sucker mouth that once rasped algae is now large and powerful enough to latch onto the slime coats of slow-moving, flat-bodied tankmates — discus, angelfish, large goldfish, plecos and similar fish are all at risk. The fish does not necessarily intend aggression; the reflex to rasp at a smooth surface can be indiscriminate. The practical result is an increasingly difficult, potentially injurious community resident, not the self-effacing glass-cleaner of its marketing.
Mating
Very little is documented about the reproductive behaviour of Gyrinocheilus aymonieri in the wild. The species spawns in fast-flowing river environments, and spawning is almost certainly tied to seasonal changes in water level, temperature and flow — the broad pattern seen across Cypriniformes in monsoon-driven Southeast Asian rivers. Adults are believed to be egg-scatterers releasing adhesive or semi-adhesive eggs among stones and gravel in fast water, with no parental investment after spawning.
Sexing live fish is not straightforward. Ripe females may be visibly plumper from the gravid belly, but outside of breeding condition the sexes are not readily distinguished by external characters. This, combined with the species' large adult size and the absence of any seasonal cue in a home tank, is part of why the species is not kept with breeding ambitions in the hobby.
Breeding
Gyrinocheilus aymonieri is not bred in home aquaria. Like most species in the trade that arrive in enormous numbers at low prices, it is produced commercially on farms in Southeast Asia using hormone induction — brood fish are treated to bring them into spawning condition, and the resulting eggs and larvae are reared under controlled conditions at scale. This farm-supplied stock is what fills aquarium shops worldwide; there is no meaningful tradition of hobbyist breeding.
The honest picture here is that an aquarium keeper should not expect to breed this fish. The combination of large adult size, aggression between conspecifics at maturity, absence of seasonally triggered breeding readiness, and a near-complete lack of documented home aquarium spawnings makes it a fish to be kept for its own sake — not as a breeding project.
In the aquarium
Gyrinocheilus aymonieri is one of the most widely sold and most widely misunderstood fish in the trade, and the typical keeper's experience follows a predictable arc: a charming, industrious little glass-cleaner that gradually becomes a large, aggressive, territorial fish that is not cleaning the glass anymore and is hassling everything in the tank. Understanding that arc before purchase is the key to keeping this species well — or to deciding not to keep it.
For those who do keep it, a few principles apply. A single specimen in a large tank (at least 59 in in length to allow it to establish a territory that does not encompass the whole aquarium) is more manageable than a group, where adults will fight. Tankmates should be fast, robust and too large to be rasped: avoid slow, flat-bodied fish and anything with delicate fins. Provide plenty of current, good oxygenation and a mature biological surface for grazing; supplement with sinking algae wafers, spirulina-based pellets and blanched vegetables, and include meaty foods as the fish grows. Remove it from any community where it is latching onto other fish — once that behaviour starts it escalates. The fish is not particularly sensitive to medications compared to the scaleless botiid loaches, but clean, well-oxygenated water is still non-negotiable. Bought by impulse for a small community tank, it is a mistake; kept with space and realistic expectations, it is a long-lived, characterful fish.
Conservation
Gyrinocheilus aymonieri has not been formally assessed on the IUCN Red List and carries no conservation category; it is recorded here as Not Evaluated. It is widespread across the Mekong and Chao Phraya drainages of mainland Southeast Asia and is not considered to be in decline, occurring over a large native range spanning Thailand, Laos, Cambodia and Vietnam.
The enormous global trade in this species is supplied almost entirely by farm-produced stock using hormone induction, so collecting pressure on wild populations is not the primary concern. The broader risks to the species and its habitat are those facing freshwater fish across the Mekong basin generally: hydroelectric dam construction (the Mekong mainstream dams are among the most significant freshwater biodiversity pressures in the world), habitat degradation, water extraction for agriculture, and changes to seasonal flood regimes that underpin the reproductive cycles of riverine fish. These pressures affect a wide community of species in the same rivers, and Gyrinocheilus aymonieri's current abundance should not be taken as a permanent condition in a basin under sustained anthropogenic stress.