This species is among a handful of near-identical congeners that are traded as 'Siamese algae-eater' (often abbreviated to 'SAE'), 'Siamese flying fox' and 'Crossocheilus siamensis'. Recent work has shown that name to be a synonym of Epalzeorhynchos siamensis which was described from Tadi province, southern (peninsular) Thailand by Smith (1931) before being moved to Crossocheilus by Bănărescu (1986).
The latter name is not valid, however, and is a synonym of Epalzeorhynchos siamensis which is itself a synonym of Crossocheilus oblongus, a species described from Java. C. oblongus is currently accepted to range throughout Indochina and the Sunda Islands but its identity is unclear although its name is routinely applied to fish in the aquarium trade. It was described as a blueish fish with yellow fins and is almost certainly not the 'SAE' in the aquarium hobby.
Separating the species found on sale as 'C. siamensis' is tricky but differences do exist if combinations of characters are considered. C. atrilimes is most-easily identified by observing the distance between the vent and anal fin which in this species is only 1.5-2 scale widths compared to 2-3.5 in other species.
The black lateral stripe runs from the snout to the tip of the caudal fin; the eye is pale brown above the pupil and white below; maxillary barbels are not visible; the lateral line is curved; the fins are a pale dusky yellow colour. Depending on mood this species has the ability to change the appearance of the dark body stripe, most obviously during bouts of sparring when it becomes much broader and paler in colour.
C. langei sensu amplo seems to be the other commonly-traded, laterally-striped Crossocheilus and can be told apart from C. atrilimes by the following characters: the eye is reddish-golden above the pupil and white below; it has two pairs of barbels; the lateral line is essentially straight and passes through the centre of the dark body stripe; the fins are brownish.
Although the body stripe becomes paler when the fish are sparring it doesn't change in width, and a further simple distinguishing trait is the presence of a dark blotch immediately in front of the anal fin on the underside of the fish. This fish is the one most often referred to as 'Siamese algae eater' in literature and online, but because it varies slightly with C. langei sensu stricto the possibility that it's an undescribed species cannot be discounted.
The third fish in the group is an undescribed species sometimes referred to as C. sp. 'citripinnis'. It's a larger fish growing to 150 mm SL and possessing a clearly visible pair of maxillary barbels. The fins are lemon yellow in colour, this becoming more intense as the fish mature; the dark body atripe doesn't alter in width or colour when the fish are sparring; the lateral line is curved and the overall body shape is similar to C. atrilmes meaning they're easily confused as juveniles. There may also be other, potentially undescribed, species from Thailand being traded as C. siamensis but as yet no detailed study has been conducted.
In 2009 Tan and Kottelata described a new laterally-striped species, C. obscurus, from the Batang Hari river drainage in Sumatra. This species also grows relatively large (to "at least" 5.6"/14.2 cm6 in) and is further distinguished as follows: "one pair of rostral barbels, no maxillary barbels; midlateral stripe with edges not sharply contrasted, slightly curved downward, obscured in largest individuals, continued on median caudal-fin rays, reaching posterior margin; no black mark between anus and anal fin; mouth wide (30-36% HL)". It's possible that this one has already appeared in the trade labelled with a different name.
C. oblongus is another name widely misused in the trade but that species has seemingly never been exported and was described as a blueish fish with yellow fins. It's native to streams of Gunung Salak mountain in Bogor Regency, West Java, Indonesia where collecting of ornamental fishes is almost non-existent. Other species of laterally-striped Crossocheilus also exist and may be available from time-to-time but are more easily told apart from the group described above.
In C. nigriloba, for example, the dark body stripe uniquely breaks up into a series of blotches when the fish are sparring, stressed or sleeping and the lower caudal fin lobe contains dark pigmentation suffused with red, while C. burmicanus exhibits a patch of blue colouration at the base of the pectoral fins.
To add further confusion the vaguely-similar Garra cambodgiensis (itself often referred to incorrectly as G. taeniata) is usually sold with the name 'false SAE' although this one is easily-identifiable as the dark lateral stripe ends at the caudal peduncle, all barbels are tiny and it has a disc-like lower jaw which sometimes develops bright red colouration on the outer lips.
Epalzeorhynchos kalopterus is sometimes misidentified as a Crossocheilus sp. but exhibits several distinguishing traits, the most obvious of which are the characteristic white-edged, red and black coloured fins. Also comparable are the Paracrossochilus species from the island of Borneo but these are almost unknown in aquaria.
Members of Crossocheilus are characterised by possessing 8 branched dorsal fin rays, immobile rostral lobes, lacking a dorsal spine and by the fact that the upper and lower lips aren't connected, the upper being attached to the lower jaw via a thin membrane.
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