Taxonomy & naming
Melanotaenia rubripinnis was described by Gerald R. Allen and Samuel J. Renyaan in 1998, in a paper titled 'Three new species of rainbowfishes (Melanotaeniidae) from Irian Jaya, Indonesia,' published in the journal aqua: Journal of Ichthyology and Aquatic Biology (volume 3, part 2, pages 69–80). The type material came from the Wapoga River system of what was then Irian Jaya — today the Indonesian province of Papua — on the northern side of New Guinea. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia rubripinnis Allen & Renyaan, 1998; because the species was described in Melanotaenia, where it remains, the authority is written without parentheses.
Melanotaenia is the largest genus in the family Melanotaeniidae, the true rainbowfishes — an Australasian radiation centred on New Guinea and northern and eastern Australia. M. rubripinnis belongs to a cluster of northern New Guinea species, and like most of its relatives it is told apart from close kin more by colour and live appearance than by countable characters, since the meristics of the genus overlap broadly. Recorded fin counts are 4 to 6 dorsal spines and 14 to 19 dorsal soft rays, with a single anal spine and 22 to 28 anal soft rays.
Morphology
Melanotaenia rubripinnis is a small to medium rainbowfish, reaching a maximum of about 4.5 in standard length according to the IUCN assessment (Allen and Kadarusman), with the original description by Allen and Renyaan based on type material up to about 4 in SL. As in all members of the genus it has the family's characteristic divided dorsal fin — a short anterior portion followed by a longer posterior one — set on a laterally compressed body that deepens with age, most noticeably in dominant males.
The name rubripinnis, 'red-finned,' points to the obvious feature: a reddish to orange tint on the fins, brightest in displaying males. As across rainbowfishes generally, colour intensity tracks mood, condition, and lighting, so a courting or dominant male flushes far more strongly than a stressed or subordinate one. Detailed colour data for this species is sparse in the sources. Beyond the red-finned character and the genus-typical body form there is little to state with confidence; females and juveniles, following the genus, should be plainer and more silvery than adult males.
Habitat
Melanotaenia rubripinnis is known from the Wapoga River system of northern Papua, Indonesia. Unlike many narrowly endemic rainbowfishes, it is reported as relatively common across a wide variety of habitats within that drainage — tannin-stained creeks running through lowland rainforest, larger streams in more open country, and mountain tributaries up to roughly 1312 ft elevation.
That breadth covers a range of flow and light: quiet shaded pools, the sunlit reaches of larger streams, and fast-flowing mountain water. Such flexibility is unusual within Melanotaenia, where many species are tied to a single lake or short stretch of river. It is the central fact of this fish's biology — the likely reason its conservation status is secure, and a sharp contrast to its more restricted relatives. As a tropical, benthopelagic freshwater fish it lives in the warm waters typical of lowland and montane New Guinea streams; precise temperature, pH, and hardness figures for its localities are not given in the sources, so treat them as data sparse.
Feeding
Detailed dietary observations specific to Melanotaenia rubripinnis are not available in the sources, but FishBase places the species at a trophic level of about 2.9, consistent with the omnivorous, invertebrate-leaning feeding typical of rainbowfishes. Members of the genus feed in the upper and middle water column on small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects taken at the surface — supplemented by algae and plant material.
In captivity rainbowfishes of this kind are undemanding feeders that take good-quality flake and small pellets alongside frozen and live foods such as Daphnia, brine shrimp, and bloodworm. A varied diet with a live or frozen component brings out the best colour and condition. Firm species-specific feeding data for M. rubripinnis is lacking, so read these as genus-level context rather than observations confirmed for this fish.
Mating
FishBase records the reproductive style of Melanotaenia rubripinnis as distinct pairing — a male and female pair off to spawn rather than spawning in an undifferentiated mass. That fits the broader rainbowfish pattern, in which a displaying male courts a female with heightened colour and fin-flaring before the pair move together into vegetation to release eggs.
Detailed courtship observations for this species are not available, so the finer behaviour — timing of display, the flickering courtship band, the preference for early-morning spawning seen in many relatives — cannot be asserted for M. rubripinnis. The sources support only that it reproduces by pairing and, like other Melanotaenia, is an egg-layer rather than a livebearer.
Breeding
No species-specific captive-breeding account for Melanotaenia rubripinnis exists in the sources, and the fish is uncommon in the hobby, so this section can only sketch the genus-typical pattern. Rainbowfishes of this group are egg-scattering plant spawners with no parental care: conditioned adults deposit small, adhesive eggs that hang by fine filaments from fine-leaved plants such as Java moss or from synthetic spawning mops.
In related species a female releases a few eggs at a time over many days rather than a single clutch, and the eggs hatch after roughly one to two weeks at normal tropical temperatures, sooner in warmer water. Adults eat their own eggs, so breeders move the spawning medium to a separate container; the tiny fry need very small first foods before graduating to newly hatched brine shrimp. None of this has been documented for M. rubripinnis — read it as the expected behaviour of the genus, not a confirmed account. The data here is sparse.
In the aquarium
Melanotaenia rubripinnis is rarely seen in the aquarium trade, and dedicated care profiles for it do not appear in the sources. Husbandry guidance therefore has to be drawn cautiously from the genus rather than from experience with this fish. Rainbowfishes of this size are active, peaceful schoolers that do best in groups, in long tanks with open swimming room, and with similarly sized, peaceful tankmates.
FishBase classes the species as medium resilience, with a minimum population doubling time of 1.4 to 4.4 years — which says nothing direct about aquarium suitability but fits a hardy, reasonably productive fish. Given the wide natural range, from tannin-stained lowland creeks to fast mountain streams, captive specimens would likely tolerate a range of conditions. Without confirmed data, the sensible approach is stable, warm, well-filtered water and a group of its own kind. Any more precise water-chemistry figure would be guesswork.
Conservation
Melanotaenia rubripinnis is assessed as Least Concern (LC) on the IUCN Red List, by Allen and Kadarusman, dated 5 December 2019 and published in 2020. The listing reflects that, despite a distribution centred on the single Wapoga River system, the species is relatively common and occupies a wide variety of habitats within that drainage — lowland rainforest creeks through to montane tributaries — which buffers it against the localized pressures that threaten more narrowly endemic rainbowfishes.
Medium resilience and broad habitat tolerance both work in its favour. Even so, a range confined to one northern Papuan river system is no guarantee against future habitat change, and ongoing monitoring is the prudent course for any rainbowfish with a limited distribution. For now the evidence supports a stable, non-threatened status.