The genus Pangio is one of the most species-rich genera in the loach family Cobitidae and is widespread across South and Southeast Asia. It is the largest genus within the southern lineages of Cobitidae, with nearly three dozen recognized species. Pangio loaches are small (<12 cm total length), elongated freshwater fishes. According to Kottelat and Lim (1993), Pangio is distinguished from all other genera of the family Cobitidae by its very slender and compressed body, by the position of the dorsal fin (which originates distinctly behind the pelvic origin rather than in front, above, or slightly behind), and by the large number of vertebrae. Mature males of all species can be easily distinguished from females by the presence of a thickened first branched pectoral fin ray. Breeding males can be recognized by the presence of a tuberculate pad on the dorsal surface of the pectoral fin, covering the distal half of the enlarged first branched ray and part of the second branched ray.
The different Pangio species inhabit a variety of habitats ranging from moderately swift to very slow flowing streams and representatives are found from India and southern Vietnam to Java. Pangio loaches live in benthic substrates, typically in submerged leaf litter and dense aquatic vegetation. Several species may occur together at the same locality and up to seven species may be found within a single river basin. They are gregarious when kept in aquaria (and presumably in nature), congregating under rocks or decaying vegetation.
Some Pangio species are collected and exported for the aquarium trade. These banded species are generally known as "kuhli loaches" based on the specific epithet of one species, P. kuhlii (named after an active collector in the Dutch East Indies, Heinrich Kuhl [1797-1821]). They are easily recognized by their bright pink to orange and black barred color pattern. They are sometimes inappropriately called "Coolie Loaches" in the aquarium literature. partly for this reason, several researchers have begun to use the common name "eel-loaches" to refer generally to all Pangio species. Kottelat and Lim (1993) reviewed the eel-loach species of the Malay Peninsula (Singapore, West Malaysia and Thailand south of the Isthmus of Kra), including comments on the systematics of species from adjacent areas.
As noted above, the Kuhli Loach (Pangio kuhlii) and close relatives are familiar fishes in the aquarium trade. Pangio kuhlii is apparently widely distributed on the Sunda shelf, except in Sarawak where it is replaced by P. agma (Burridge, 1992). This distribution pattern is shared with several other pairs of fish species (Kuttelat and Lim 1993). Pangio kuhlii is known from West Java, Sumatra, Kalimantan Timur and Kalimantan Barat (Borneo), and the Malay Peninsula at least as far north as Phangnga (records from Burridge  from central and southeastern Thailand were considered questionable by Kittelat and Lim , who suggested they may actually refer to P. myersi). Pangio kuhlii has 6 to 10 usually irregular bars along its body with a dark large quadrangular blotch occupying the proximal half of the caudal (tail) fin; in contrast to some other Pangio species, the median lobe of the lower lip is not produced into a barbel; and the vertebral count is 34 to 37+ 12 to 15 = 47 to 51 vertebrae.
Based on the molecular phylogenetic analysis by Bohlen et al. (2011), there are three main lineages within the genus Pangio, two of which correspond to two of the four groups recognized by Kottelat and Lim (1993), with the third group matching the remaining two groups combined. The kuhlii-group includes all species with a dark brown or black banding pattern on yellow or red background, including P. kuhlii.
Burridge (1992) provided technical descriptions of the genus and analysis of the P. kuhlii complex.
(Burridge 1992; Kottelat and Lim 1993; Bohlen et al. 2011 and references therein)
Bohlen, J., V. Šlechtová, H.H. Tan, and R. Britz. 2011. Phylogeny of the Southeast Asian freshwater fish genus Pangio (Cypriniformes; Cobitidae). Molecular Phylogenetics and Evolution 61: 854-865.
Burridge, M.E. Systematics of the Acanthophthalmus kuhlii Complex (Teleostei: Cobitidae), with the Description of a New Species from Sarawak and Brunei. Copeia 1992(1): 172-186.
Kottelat. M. and K.K.P. Lim 1993. A review of the eel-loaches of the genus Pangio (Teleostei, Cobitidae) from the Malay Peninsula, with descriptions of 6 new species. Raffles Bulletin of Zoology 41(2): 203-249.