Taxonomy & naming
Kadarusman, Sudarto, Paradis and Pouyaud described the species in 2010, in the journal Cybium (volume 34, pages 207–215) — a paper that also commented on the rediscovery of M. ajamaruensis and the endangered status of M. parva. The type material comes from the Bird's Head (Vogelkop) Peninsula of West Papua, Indonesia, in northwestern New Guinea, a hotspot for the genus where many species hold very small ranges. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia fasinensis Kadarusman, Sudarto, Paradis & Pouyaud, 2010; because it was described in Melanotaenia, where it remains, no parenthetical authority is used.
The genus name combines the Greek melan (black or dark) with the Latin/Greek taenia (stripe or band) — a nod to the dark lateral band carried by many members of the group. The epithet fasinensis is geographic, naming the species for its type locality at Ween village on Fasin Creek (Sungai Fasin), the river behind its English name, the Fasin rainbowfish. Melanotaenia is the largest genus in the family Melanotaeniidae, the true rainbowfishes and blue eyes, which radiate across New Guinea and northern and eastern Australia within the order Atheriniformes.
Morphology
A moderately deep-bodied, laterally compressed rainbowfish, M. fasinensis reaches a maximum recorded length of about 4.5 in SL. It carries the family's divided dorsal fin — a short anterior dorsal followed by a longer posterior one — set against a body that deepens with age in well-conditioned fish.
Colour is the species' calling card. Importers describe its rapid, frequent colour change as 'chameleon-like', and breeding males develop a deep red-orange flush that makes them among the most sought-after of the Bird's Head rainbowfishes. Body form is awkward for sexing: unlike many Melanotaenia, where only males go high-backed, this species includes both slender and high-backed males alongside high-backed females, so body depth alone won't tell you sex. Freshly caught and stressed fish lose their colour and look drab, which makes the sexes even harder to tell apart at first — full colour and form arrive only once the fish settle and come into condition.
Habitat
The species is endemic to the Bird's Head (Vogelkop) Peninsula of West Papua, Indonesia, tied to the karst — limestone — landscape that dominates much of the region. The type locality lies at Ween village, where small streams feed the Fasin River, part of the larger Kladuk River system of the central Bird's Head; these streams are spring-fed and drain mostly underground after a short surface run. The IUCN assessment describes the habitat as clear, spring-fed streams of gentle gradient through secondary forest, with the type specimens taken from a narrow creek 13–16 ft wide and up to about 3 ft deep, over a bottom of gravel and limestone boulders littered with dead branches. Karst catchments tend to give clear, mineral-rich, hard and alkaline water — a pattern shared by several other rainbowfishes of this limestone region.
The narrowness of the range is the central fact of the species' biology. The Bird's Head is a hotspot where most Melanotaenia hold tiny ranges and where collection and travel are difficult, so the fish is known from only a short stretch of water. That restriction drives both its rarity in the hobby and its conservation status. No in-situ water measurements for the type locality appear in the sources consulted, so wild temperature and pH figures are data sparse and are not asserted.
Feeding
No detailed wild dietary study for M. fasinensis appears in the sources consulted, but it can be expected to follow the genus. Rainbowfishes are opportunistic omnivores that feed in the upper and middle water column, taking small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, insects off the surface — along with algae and plant material grazed from submerged surfaces. FishBase puts the species at a trophic level of roughly 2.9, consistent with a largely omnivorous diet that leans toward small animal prey.
In the aquarium, treat it like any other Melanotaenia: an undemanding feeder that takes good-quality flake and small pellets alongside frozen and live foods such as Daphnia, brine shrimp, bloodworm and Cyclops, with some vegetable matter. A varied diet with a real live or frozen component improves both colour and breeding condition, and as an active grazer the fish does best on small amounts several times a day rather than one large meal.
Mating
Like rainbowfishes generally, M. fasinensis is expected to be an egg-scattering, plant-spawning species with no parental care, spawning more or less continuously under good conditions rather than in one seasonal event. Males court with intense colour displays, and here the courting male's red-orange flush is the centrepiece — the same rapid colour change that makes the fish a hobby favourite doubles as reproductive signalling.
Sexing for breeding is genuinely tricky, because body depth doesn't reliably split the sexes: both slender and high-backed males occur, females may also be high-backed, and stressed or freshly imported fish are washed out and hard to read. Breeders tend to condition a group and watch for displaying, high-colour males rather than picking sexed pairs by shape alone. Courtship observations specific to M. fasinensis are sparse, so the account here rests on genus-level behaviour rather than published species-specific study.
Breeding
The species is reported to be hard to breed on a large scale, which — together with the difficulty of collecting it in the Bird's Head — keeps it rare and expensive in the trade. Compare easier congeners such as M. boesemani, where abundant tank-bred stock has made the fish commonplace.
Where it does spawn, expect the rainbowfish template: conditioned adults deposit small, adhesive eggs that hang by fine filaments from fine-leaved plants such as Java moss or from synthetic spawning mops, a female releasing a few eggs daily over a long period rather than one large clutch. Adults give no care and will eat eggs and fry, so breeders lift the mops to a separate rearing container; the tiny fry need very small first foods such as infusoria or liquid fry food before graduating to newly hatched brine shrimp. Figures for egg counts, incubation time and fry growth specific to this species are data sparse in the sources consulted and are not asserted here.
In the aquarium
This is a specialist's rainbowfish, not a beginner's fish — not because it's delicate, but because it's rare, costly, hard to breed in quantity, and confusing to sex. Keep it as other rainbowfishes are kept: an active, peaceful schooling fish in a group of six or more, in a tank long enough to give these restless swimmers an open lane. At around 4.5 in, an adult group wants an aquarium well over a metre long.
The importer sources publish no tank parameters for this species, but its karst, limestone origin argues for neutral to alkaline, moderately hard and warm water rather than the soft-water default often assumed for tropical fish — in line with other Bird's Head rainbowfishes. Stable conditions, good filtration, bright lighting and a planted layout with open central swimming space show the colours best and bring on the male's display. Since freshly imported fish lose colour and look drab, give them time and conditioning to reach full red-orange form rather than judging them on arrival.
Conservation
The IUCN Red List assesses M. fasinensis as Endangered (EN) under criterion B1ab(iii), the assessment dated 4 December 2019. That criterion flags a small extent of occurrence with a continuing decline in habitat quality — what you'd expect of a narrow-range Bird's Head endemic with little room to absorb local pressure. CITES and CMS list the species as not evaluated.
The assessment documents two main threats. The first is introduced fishes: invasive common carp (Cyprinus carpio) and Nile tilapia (Oreochromis niloticus) are already in the catchment, and hatcheries for Nile tilapia and goldfish (Carassius auratus) operate nearby, raising the odds of further spread. The second is land-use change — an oil palm plantation lies 4–6 mi from the type locality, and the assessment projects a continuing decline in habitat quality as it expands. Confined to a single small karst catchment (an extent of occurrence of about 81 mi², known from one location), the species has no large outlying population to buffer such impacts. Unlike the easily tank-bred M. boesemani, it is reported as hard to breed on a large scale, so captive stock does little to relieve pressure on the wild population; responsible keepers should seek out what tank-bred fish exist and avoid driving demand for wild collection.