Every species

Microdevario gatesi is a miniature cyprinid from the lower Irrawaddy basin in Myanmar, growing to around 23 mm SL. Uncommon in the aquarium hobby, it is sometimes traded as 'Burmese golden rasbora' and can be told from its congeners by its hyaline dorsal and anal fins. It is likely to inhabit sluggish and standing waters such as rice paddies, swamps and weedy ditches, and is possibly endemic to the Irrawaddy Delta.

Microdevario kubotai is a miniature cyprinid from peninsular Thailand and the Salween drainage in southern Myanmar. Relatively common in the ornamental trade, it is sold under names including 'green neon rasbora' and 'yellow neon rasbora'. It apparently inhabits calm to moderately-flowing, well-oxygenated headwaters and minor tributaries. In nature it typically occurs in groups of 20-50, and at least 8-10 are recommended.

Microdevario nanus is a miniature cyprinid from the lower Sittaung and Bago drainages of the Irrawaddy system in Myanmar. Sometimes referred to as 'green dwarf rasbora' or 'nana rasbora' in the ornamental trade, it grows to around 16 mm SL. A black blotch at the tip of the dorsal fin and a greyish marking at the tip of the anal fin immediately distinguish it from congeners.

Micronemacheilus cruciatus is a small nemacheilid loach that appears to be endemic to the coastal rivers of Vietnam. It is traded under names including 'Vietnamese multi-banded zebra loach' and 'hovering zebra loach', and its colour pattern is a dark lateral stripe with numerous vertical cross-bars. It can be susceptible to swings in water chemistry and should not go into a biologically immature aquarium.

The flame red 'rasbora', Microrasbora cf. rubescens, is a small cyprinid from Shan State in eastern Myanmar. Possibly an undescribed species, it has been marketed as 'Asian Cardinal Rasbora', M. sp. 'rose blue line' or M. 'thuzari'. It differs from M. rubescens in its larger adult size, a greenish to bluish lateral stripe and less orange on the body, and appears to represent a distinct genetic lineage.

The red dwarf 'rasbora', Microrasbora rubescens, is a small cyprinid endemic to Lake Inle and its watershed in Shan State, eastern Myanmar. It is listed as 'Endangered' on the IUCN Red List after a significant decline in the wild, as sedimentation, eutrophication, pollution and introduced predatory Parambassis and Tilapia have degraded the lake. It may also be sold as 'red-line dwarf rasbora' or 'emerald dwarf rasbora'.

The dwarf synodontis, Microsynodontis batesii, is a small catfish from the Congo and Ogowe basins in DR Congo, Gabon and Cameroon. The largest Microsynodontis, it is sometimes mislabelled as Synodontis sp. 'Nyong'. Young fish are sold with a spotted or reticulated pattern that breaks up into the characteristic barring as they grow, and new imports are often delicate. At least some of its recorded occurrences may be different species.

The pygmy synodontis, Microsynodontis polli, is a small catfish from the upper Niger basin in Liberia and Guinea, growing to around 40 mm SL. It is sometimes sold mislabelled as Synodontis sp. 'Nyong' or M. batesii but lacks the distinct body barring of that species. New imports are often delicate, and it is quite shy, so it should not be kept with aggressive or very vigorous fish.

The Bolivian ram, Mikrogeophagus altispinosus, is a cichlid endemic to the upper rio Madeira basin in Bolivia and Brazil. Also traded as 'hi-fin ram' and 'Bolivian butterfly', it is larger and less colourful than M. ramirezi and lacks its blue iridescent markings, and it is arguably a better choice for less-experienced aquarists. A soft, sandy substrate is essential, and it needs pristine water quality.

The ram, Mikrogeophagus ramirezi, is a dwarf cichlid from the llanos of the Rio Orinoco drainage in Venezuela and Colombia. Among the more popular dwarf cichlids in the hobby, it is produced commercially in huge numbers, and ornamental strains such as 'gold', 'long-finned' and 'balloon' tend to be genetically weak and short-lived. Wild fish are of superior stock but more demanding, and it requires pristine water quality.

The blue tetra, Mimagoniates microlepis, is a characid from southeastern Brazil, found from southern Bahia to northern Rio Grande do Sul. Not to be confused with Knodus borki, also sold as 'blue tetra', it is much scarcer in the hobby, less boisterous and lacks that species' tendency to nip fins. It inhabits flowing clear-water streams and small rivers of coastal floodplains, and is a predominantly surface-dwelling fish.

The oriental weather loach, Misgurnus anguillicaudatus, is a loach native to Siberia, Korea, Japan, China and northern Vietnam. Known in the United States as the 'dojo loach', it is one of the most ubiquitously available species in the hobby, and years of mass-production have made health and morphological issues common. Exceptionally hardy and adaptable, it is established as an alien species around the globe, and its adult size of up to 280 mm should be considered prohibitive.

The large-scaled loach, Misgurnus dabryanus, is a 'true' loach from the Yangtze and Pearl rivers in China, plus Taiwan and the Amur in Russia. It is sold regularly in the hobby but almost always labelled as or mixed in with M. anguillicaudatus, from which it differs in its high adipose crests on the caudal peduncle and the lack of a dark spot at the caudal-fin base. It spends some of its time completely buried, so it needs a soft, sandy substrate.

The ceros tetra, Moenkhausia ceros, is a small characid from the Amazon basin in Peru and Brazil. Rare in the UK hobby, it occasionally shows up as a contaminant among imports of more popular species. It inhabits slow-moving rivers, tributaries and floodplain lakes. Moenkhausia is used as a catch-all for many small characins, and most experts agree the genus needs a full revision.

The southern purple spotted gudgeon, Mogurnda adspersa, is an Australian gudgeon considered endangered across its native range. Commercially farmed fish reach the hobby. It looks very similar to the commoner northern purple spotted gudgeon, M. mogurnda, but has only 30-36 lateral line scales (37-48 in M. mogurnda) and more red spotting in the second dorsal fin. Territorial but not particularly boisterous, it will probably eat any tankmate it can fit in its mouth.

The northern purple spotted gudgeon, Mogurnda mogurnda, is a gudgeon from Australia and Papua New Guinea. Tolerant of a wide range of water conditions, hardy and easily bred, it suits the beginner, and it can survive low oxygen in habitats from forest streams to desert boreholes. It is territorial and known to nip fins, so should not be kept with very delicate or long-finned species. It is easily confused with the rarer southern purple spotted gudgeon, M. adspersa.

The South American leaf fish, Monocirrhus polyacanthus, is a predator from Brazil, Peru, Venezuela, Colombia and Bolivia that mimics a dead leaf. It stalks prey head-down, appearing to drift like a leaf, and can swallow prey almost as big as itself. Flighty and very sensitive to deteriorating water quality, it is difficult to maintain and not a species for the beginner, and soft, acidic water is essential.

The swamp eel, Monopterus albus, is very widespread, found from India and China to Japan, Indonesia, the Philippines and Australia. Not a true eel, it has a single gill slit and an accessory breathing organ, and during dry spells it burrows into wet earth. Growing to around 100 cm, nocturnal and piscivorous, it is best kept singly, and the tank must have an extremely tightly-fitting, heavy cover.

Mustura maepaiensis is a small nemacheilid loach from tributaries of the Salween basin in Mae Hong Son and Tak provinces, northwestern Thailand. It may also occur in the Salween system in Myanmar. Poorly known in the hobby, it can be told from congeners by its 10-13 irregular body bars with thin dark borders. The type locality was a shallow stream over rocks and boulders with moderate to swift flow.

Known in the trade as 'Haplochromis yellow-black line', Mylochromis melanonotus is a predatory cichlid endemic to Lake Malawi. Rarely imported, it caused confusion because some individuals have a shovel-like lower jaw. Ad Konings found wild populations with and without it, and Platygnathochromis and M. semipalatus are now regarded as synonyms. A sandy substrate is essential, and it will eat small fish.