Taxonomy & naming
Crossocheilus oblongus was described in 1823 by Heinrich Kuhl and Johan Coenraad van Hasselt, the young Dutch naturalists whose Java expedition named a remarkable share of the island's freshwater fauna before both died in the field. It is the type species of Crossocheilus, a genus of slender, ground-hugging labeonine cyprinids (family Cyprinidae, subfamily Labeoninae) whose name — Greek krossoi, fringe, and cheilos, lip — describes the fringed upper lip of the grazing mouth. Eschmeyer's Catalog of Fishes treats Crossocheilus oblongus Kuhl & van Hasselt 1823 as valid, with a long synonymy that includes Labeo oblongus Valenciennes 1842 and, most consequentially for aquarists, Epalzeorhynchus siamensis Smith 1931, described from the Tadi River in peninsular Thailand.
Smith's siamensis is the name under which the Siamese algae eater conquered the hobby, and 'Crossocheilus siamensis' persists on price lists decades after Roberts (1989) and later Kottelat placed it in the synonymy of oblongus. The working confusion runs deeper still: revisions by Tan & Kottelat (2009) and the genus review of Ciccotto et al. (2017) show that fish traded as Siamese algae eaters include the very similar Crossocheilus langei and other congeners such as C. atrilimes, and that the true bearer of the name oblongus in the strict, Javan sense may be rare in the trade. The atlas follows the Catalog and treats the trade complex under the valid name C. oblongus, flagging the identification caveats where they matter.
Morphology
The Siamese algae eater is a slender, low-slung cyprinid reaching about 6.5 in standard length, though most aquarium fish stay under 4.5 in. The body is elongate and only slightly compressed, pale sandy-brown to olive above and whitish below, with each dorsal scale dark-edged to give a faint netted sheen. The defining mark is the midlateral stripe: dead black, running unbroken from the tip of the snout through the eye and body and — critically for identification — continuing onto the caudal fin to the fork's end, with a distinctly ragged, zigzag upper edge where it meets the pale flank. The remaining fins are clear to faintly hyaline, never washed with yellow or edged in white.
Those details separate it from its perennial impostors. The flying fox (Epalzeorhynchos kalopterus) has a smooth-edged black stripe topped by a gold band and boldly coloured, white-tipped fins; the 'false siamensis' (Garra cambodgiensis) has a stripe that stops at the caudal-fin base and dark-banded dorsal fin. Within Crossocheilus, C. atrilimes is smaller with the stripe barely entering the tail, and C. langei — likely much of the farmed trade stock — differs in lip fringe and snout details that are honestly beyond shop-tank identification. One pair of small maxillary barbels is present, and the mouth is the group's underslung grazing apparatus with a fringed upper lip.
Habitat
Crossocheilus oblongus in the broad sense ranges across mainland Southeast Asia and Sundaland: the Mekong basin in Thailand, Laos and Cambodia, the Malay Peninsula, and the Greater Sunda islands, with the type locality at Bogor in western Java. It is a fish of clear, flowing water — hill streams, river reaches near rapids, and stony runs where sunlight reaches the substrate and keeps the aufwuchs mat growing on boulders, bedrock and submerged timber. FishBase records it over a pH envelope of roughly 6.5–8.0, hardness to about 20 °dH, and temperatures near 75–79 °F.
Like many stream labeonines it holds station near the bottom, propped on its pectoral fins between grazing bouts, and moves in loose feeding aggregations rather than tight schools. Populations in the Mekong system undertake seasonal movements with the flood pulse, shifting between mainstem refuges and flooded feeding grounds. The species' whole design — flattened belly, inferior mouth, low-slung profile — reads as an animal that makes its living scraping surfaces in current, and its aquarium requirements follow directly from that: oxygen-rich, moving water over hard, grazeable structure.
Feeding
The natural diet is aufwuchs in the broad sense: filamentous and encrusting algae, diatoms, biofilm, and the micro-invertebrates and organic detritus bound up in the mat, all rasped from hard surfaces with the fringed lips. The species' fame rests on one particular item — it is among the very few aquarium fishes that consistently eat red algae of the genera Audouinella and Compsopogon, the 'black brush' and 'staghorn' algae that resist almost every other grazer. That reputation, established in the hobby literature of the 1970s–80s and repeated with caveats ever since, is genuine but conditional: well-fed adults graze algae far less enthusiastically than lean juveniles.
In the aquarium the Siamese algae eater takes everything offered — algae wafers, sinking pellets, flake, blanched vegetables, live and frozen invertebrate foods — and given the chance will happily abandon algae for richer fare. Keepers who want working fish limit supplementary feeding and keep the group slightly hungry. The species will also eat soft-leaved plants and fine mosses when algae runs short, a habit worth knowing before adding it to a carpeted aquascape.
Mating
Reproductive behaviour in the wild is essentially undocumented in the accessible literature. The species is understood, from its anatomy and from what is known of related stream cyprinids, to be a seasonal egg-scatterer: ripe adults shed small, non-guarded eggs into open water or over the substrate, timed to the rising water and increased flow of the wet season, with no pair bond and no parental care of any kind. No courtship sequence has been formally described.
Sexual dimorphism is correspondingly modest. Ripe females are unmistakably fuller through the belly and often slightly larger; males stay slimmer. Some keepers report small tubercles on the snouts of conditioned adults, consistent with other cyprinids, but there is no reliable external character for sexing shop-sized juveniles. In the aquarium the species shows none of the territorial spawning behaviour familiar from Epalzeorhynchos; social friction among adults amounts to chasing around feeding sites and dominance sparring within groups, which settles once a hierarchy is established in adequate space.
Breeding
There is no verified account of the Siamese algae eater breeding spontaneously in a home aquarium. The supply chain runs instead through commercial farms, chiefly in Thailand, which produce the species — or the species complex, since farm broodstock has never been critically identified in the scientific literature — by hormone-induced spawning, the standard technique for river cyprinids that will not ripen and release eggs under pond conditions. Injected adults are stripped or allowed to scatter, and the eggs hatch in incubation tanks with fry raised on green water, rotifers and Artemia nauplii.
For the hobbyist this has two practical consequences. First, essentially every fish in the shops is captive-bred, so the trade places no collection pressure on wild populations. Second, the farm bottleneck is where the identity tangle perpetuates itself: whatever species the original broodstock represented — C. langei by most recent accounts, true C. oblongus by the older ones — is what the global hobby receives under the Siamese algae eater name. An aquarist who wants to attempt the species would be working from first principles: a large group conditioned heavily, a flow-and-temperature manipulation mimicking the onset of the wet season, and realistic expectations, since even public aquaria have not published a documented tank spawning.
In the aquarium
The Siamese algae eater is one of the hobby's genuinely useful fishes, and its husbandry is straightforward if its nature is respected. It is an active, streamlined swimmer that patrols constantly and grows to a size — 4.5 in and up — that surprises buyers of 1.5 in juveniles; a tank of 65 US gal or more with a metre of length suits adults. Water should be clean, oxygen-rich and moving: 75–79 °F, pH roughly 6.5–7.5 in the community context, with hardness anywhere in the soft-to-moderately-hard band. A tight lid is mandatory; these are accomplished jumpers.
Kept singly it does its job quietly; kept in groups of five or more it is more natural and more entertaining, spreading dominance interactions thin. It is peaceful toward unrelated tankmates, though its restlessness can wear on sedate fish, and adults sometimes develop a taste for the slime coat of flat-bodied cichlids and slow long-finned fish — worth watching. The classic placement is the planted or hardscape community with a persistent brush-algae problem: juveniles mow Audouinella off wood, leaves and equipment with real efficiency. Buyers should check the stripe (into the tail, ragged edge, no gold band, clear fins) to avoid taking home a flying fox or 'false siamensis' that will do a different and lesser job.
Conservation
The IUCN Red List assesses Crossocheilus oblongus as Least Concern (Vidthayanon, 2012; assessed 29 March 2011), reflecting a wide range across the Mekong basin, the Malay Peninsula and the Greater Sundas and no evidence of range-wide decline. The assessment predates the sharpened species-limits work of Ciccotto et al. (2017), so its range concept is the broad, inclusive one; if the strict Javan oblongus were ever restricted to Sundaland streams, its status would deserve a fresh look, since the island rivers of Java in particular are among Southeast Asia's most heavily modified.
The general pressures on its habitat are those of the region: deforestation of stream catchments, which silts the sunlit rock surfaces the species grazes; damming and flow alteration in the Mekong system; and pollution in lowland reaches. The aquarium trade itself exerts little pressure, being supplied by farm production rather than wild collection. The species carries no CITES listing and is not a CARES priority. Its conservation profile, like its identity, is best read at the genus level: a healthy guild of Crossocheilus in a stream is a marker of clear, flowing, sunlit water — the first thing catchment degradation takes away.