Australia

The olive flathead gudgeon, Butis amboinensis, is a nocturnal ambush predator from the eastern Indian Ocean to the West Central Pacific. It inhabits brackish estuaries, lagoons and mangroves, entering tidal freshwater rivers to an extent, and often aligns itself with solid surfaces or swims upside-down. It is occasionally exported as bycatch and differs from congeners in lacking enlarged outer teeth and scales on the cheek. It is territorial and should probably be kept singly.

The crazy fish, Butis butis, is a flathead gudgeon of coastal and brackish waters across the Indo-Pacific, from eastern Africa to Fiji. The most commonly traded member of the genus, it has many other names, including crocodile fish, duckbill sleeper and upside-down sleeper. A nocturnal ambush predator, it often swims upside-down, and it is territorial and not sociable with conspecifics, so is best kept singly.

The desert goby, Chlamydogobius eremius, is a small goby endemic to the Lake Eyre drainage basin in South Australia. It does not occur in the salt lake itself but inhabits isolated freshwater springs, flowing artesian bores and ephemeral waterholes. Males are much more colourful than females but can change colour considerably, and they can be aggressive in spawning condition, so it is best maintained alone.

The empire gudgeon, Hypseleotris compressa, is an eleotrid from northern and eastern Australia and southern New Guinea. Breeding is rarely achieved in aquaria because of its complex amphidromous strategy: adults spawn in freshwater streams, but the larvae are washed downstream to the sea and develop there. Plenty of cover and broken lines of sight let weaker individuals escape the attention of dominant fish.

The threadfin rainbowfish, Iriatherina werneri, is a small rainbowfish from southern New Guinea, Cape York in Queensland and the Northern Territory. Although widely traded and produced on a commercial scale, it is quite delicate with rather specific care requirements and is not a recommended beginner's fish. Easily outcompeted for food, and with long-finned males that some common species nip, it is best kept alone or with fishes of comparable size and disposition.

The shortfin tandan, Neosilurus brevidorsalis, is a plotosid catfish restricted to the tips of northern Australia and southern Papua New Guinea. In Australia it is recorded only from the Jardine and Jackson river systems on the Cape York Peninsula, and in Papua New Guinea from the Fly River and some other drainages.

The neon blue-eye, Pseudomugil cyanodorsalis, is a euryhaline blue-eye from coastal northern Australia and possibly southern New Guinea. Males are told apart from the rest of the genus by the bright metallic blue upper portion of the body, the source of the alternative name 'blueback blue-eye'. It has been recorded from full marine conditions to pure fresh water, and is generally considered to do better in brackish water. Timid and easily outcompeted, it is unsuitable for the general community.

The spotted blue-eye, Pseudomugil gertrudae, is a pseudomugilid from northern Australia, Papua New Guinea and the Aru Islands, eastern Indonesia. Wild populations are isolated from one another and differ in colour pattern, body size and fin morphology, but the fish on sale are of a single, commercially produced ornamental strain. Dark spots in the dorsal, anal and caudal fins separate it from other members of the genus except P. paskai and Pseudomugil sp. 'red neon'.

The Pacific blue-eye, Pseudomugil signifer, is the most widely-distributed blue-eye in Australia, found along most of the eastern coastline. Its habitats range from full marine conditions to pure fresh water, and northern populations often grow considerably larger than southern ones. Recent unpublished analyses apparently suggest it may comprise two or more distinct taxa, and wild fish from the northern and southern ends of its range would not hybridise in captive experiments.

The delicate blue-eye, Pseudomugil tenellus, is a euryhaline blue-eye with a patchy but wide range in coastal northern Australia. Sometimes called the 'Arnhem Land blue-eye', it is most easily identified by the midlateral scale row, a series of pale spots with darker spaces between that become more conspicuous with age. It is particularly common in densely-vegetated swamps, billabongs and slow-moving streams, and also occurs on the Aru Islands and in southern Papua New Guinea.

The green swordtail, Xiphophorus hellerii, is a livebearer native to southeastern Mexico, Guatemala, Belize and Honduras. Among the world's most recognisable aquarium fishes, it is ubiquitous in the trade, where selective breeding and hybridisation with X. maculatus and X. variatus have produced a bewildering array of ornamental forms. The name is often misspelled X. helleri.

The dusky millions fish, Phalloceros caudimaculatus, is a small livebearer native to Brazil, Paraguay, Uruguay and Argentina. Feral populations in Australia, New Zealand, Ethiopia and Malawi were purportedly introduced for mosquito control and are now considered invasive. Highly variable in colour pattern, it is known in the hobby in spotted and golden ornamental forms. P. caudimaculatus is the type species of a genus that was monotypic until 21 new species were described in 2008.

The fly-specked hardyhead, Craterocephalus stercusmuscarum stercusmuscarum, is an atherinid endemic to Australia. It ranges from the Northern Territory to southern Queensland and forms large shoals over barren sand, mud or clay in streams, rivers, lakes and ponds. Skittish and rarely seen in the trade outside Australia, it does far better in a shoal of at least 8-10. Genetic differences between neighbouring populations are in some cases large enough to suggest several distinct species.