Nam Theun

The dwarf snakehead, Channa gachua, is a snakehead recorded across an enormous range from Iran to Taiwan and Bali. There is significant evidence that C. gachua as currently understood is a complex of similar-looking species in need of taxonomic review. A mouthbrooder bred regularly in captivity, it can share a well-chosen community with peaceful fishes too large to be prey, but populations should not be mixed as some may hybridise.

Garra fasciacauda is a cyprinid native to the Mekong river system in Thailand, Laos, Vietnam and Cambodia, including the Tonlé Sap basin. It has a black submarginal stripe on both caudal lobes and a plain dorsal fin. It inhabits swiftly-flowing water and migrates seasonally, with many fish moving from the Tonlé Sap system to the Khone Falls in the dry season.

Hypsibarbus vernayi is a cyprinid widespread in mainland Southeast Asia, from the Mekong basin in southern China to northern Peninsular Malaysia. Very similar to H. wetmorei, it has yellow pectoral, pelvic and anal fins (orange to red in H. wetmorei), so yellow-finned fish sold as H. wetmorei or 'lemon fin barb' appear likely to be this species. It prefers flowing water with abundant riparian vegetation and migrates seasonally.

Mystacoleucus greenwayi is a riverine cyprinid from the middle Mekong basin in Thailand and Laos and the Chao Phraya watershed. It favours clear, well-oxygenated running water over sand or gravel, often in rock and boulder-filled headwater streams. A schooling fish, it should be kept in a group of six or more alongside similarly-sized cyprinids, loaches and other riverine fishes, and like all Mystacoleucus it has a forward-pointing predorsal spine that can snag in a net.

Traded as 'butterfly barb' or 'mackerel barb', Opsarius pulchellus is a cyprinid from the Chao Phraya and mid-to-lower Mekong basins. It occurs in Thailand, Laos, Cambodia and Vietnam, typically in submontane headwaters and fast-flowing minor rivers and streams. Also sold as 'royal opsarius', it carries 7-10 elongate vertical bars on each flank, and like its congeners is too fast and vigorous a feeder for the general community.

Oryzias mekongensis, the Mekong ricefish, is endemic to the Mekong River system in northeastern Thailand, Laos and Cambodia. It reaches about 21 mm SL and is known only from fresh water, mainly clear water swamps, canals, ditches and ponds with dense fine-leaved plants. Males have bright orange-red marginal bands on the caudal-fin lobes, and it may be sold as the 'Mekong redfin lampeye killifish'.

The sparkling gourami, Trichopsis pumila, is a small labyrinth fish from the lower Mekong basin and central and southern Thailand. Also known as the pygmy gourami, it has a solid dark midlateral stripe topped by a row of dark blotches, which with its small size separates it from T. schalleri. Both sexes produce audible 'croaking' sounds with a specialised pectoral mechanism, and most traded fish are produced commercially.

The royal featherback, Chitala blanci, is a very large notopterid endemic to the Mekong drainage in Laos, Thailand, Vietnam and Cambodia. Also sold as the 'royal knifefish', it reaches 1200 mm SL and arguably has no place in the ornamental hobby, being suitable only for public installations or the very largest private aquaria. Many small dark spots on the front of the body merge into oblique stripes, and it is thought to be threatened by dam construction.

Mekongina erythrospila is a large cyprinid endemic to the middle and lower Mekong River system in Laos, Thailand, Cambodia and Vietnam. It tends to inhabit medium to large river channels with fast-flowing water and rocky beds. Occasionally in the ornamental trade, it grows to 400 to 450 mm and is unsuitable for most private aquaria. It can be aggressively territorial towards conspecifics and similarly-shaped fishes.

Garra theunensis is a large cyprinid from the Nam Theun and Nam Gnouang systems of the Mekong watershed in Laos, and the Nan River in Thailand. It grows to 300 mm SL and has a plain brown body and fins, sometimes with 6-8 faint longitudinal stripes. It tends to inhabit swiftly-flowing headwaters and tributaries with clear, oxygen-saturated water.