Taxonomy & naming
Melanotaenia parva was described by the Australian ichthyologist Gerald R. Allen in 1990, from specimens collected in Lake Kurumoi (Danau Kurumoi) on the Bird's Head Peninsula of West Papua, Indonesia. The type series spanned roughly 1–2 in standard length. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia parva Allen, 1990; because the species was described in Melanotaenia, where it remains, no parenthetical authority is used. FishBase carries it under SpecCode 10514, in the order Atheriniformes and the family Melanotaeniidae, the true rainbowfishes.
The specific epithet 'parva' is Latin for 'small', for the modest adult size of the fish against many of its congeners. Melanotaenia is the largest genus in the family and ranges widely across New Guinea and northern and eastern Australia; M. parva belongs to the New Guinea assemblage centred on the Vogelkop region. At the time of description Allen already noted that the lake's water level was dropping dramatically and had receded far from its natural outlet into the Yakati River — a warning, recorded in the original account, of the precariousness that has since defined the species.
Morphology
Melanotaenia parva is a small, ovoid, laterally compressed rainbowfish. It is one of the more compact members of its genus: FishBase gives a maximum of about 2.5 in standard length, and the IUCN assessment notes a maximum of about 2.5 in SL, with a typical adult length nearer 2 in. As the epithet implies, it sits toward the smaller end of the genus. Like other Melanotaenia it carries the family's divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal — and the characteristic deep-bodied rainbowfish profile.
Body depth is modest for the genus and shows a mild sexual difference: in adult males over about 2 in SL it runs roughly 32 to 37 percent of standard length, in females about 30 to 33 percent, so mature males sit a little deeper-bodied. The colouration is subdued next to the reds and blues of better-known rainbowfishes such as M. boesemani. This is an understated fish, and live-colour descriptions are sparse in the sources — rather than guess, we leave the pattern undescribed where the record does not support it.
Habitat
The species is endemic to Lake Kurumoi (also rendered Kuromai or Kurumoi), a single small lake of roughly 500 by 800 metres on the Bird's Head Peninsula of West Papua, Indonesia, together with its presumed outlet stream. The lake lies within the Yakati River watershed. Few rainbowfishes are confined so tightly — this one is known, in effect, from a single lake.
In its natural state the lake carried dense vegetation along the shoreline, and M. parva was collected there in abundance, shoaling among the submerged plants. That habitat has since been badly altered. The IUCN assessment documents changes in vegetation and sediment and a sharp drop in water level — the surface has fallen below the natural outlet — driven largely by an introduced tilapia infestation that reworked the lake's ecology. Aquarium guidance built from the natural setting points to warm, roughly neutral water (pH near 7.0) as an approximation, but field measurements of temperature and chemistry are not well documented.
Feeding
Detailed wild-diet studies for Melanotaenia parva are not available in the consulted sources, but its biology and the general ecology of the genus indicate an opportunistic omnivore that forages in the upper and middle water column among aquatic vegetation. Rainbowfishes of this type typically take small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects from the surface film — alongside algae and plant material grazed from submerged surfaces.
In the aquarium the species is undemanding and accepts standard rainbowfish fare: good-quality flake and small pellets supplemented with frozen and live foods such as Daphnia, brine shrimp, and bloodworm, plus some vegetable matter. A varied diet with a live or frozen component improves condition and breeding readiness, and several small feedings through the day suit a constantly active grazer better than one large meal.
Mating
Melanotaenia parva is an egg-scattering, plant-spawning fish with no parental care, and both sexes are needed to spawn. There is no mouth-brooding, no live-bearing, and no nest-guarding; the Australian government's draft import-risk assessment explicitly records that the species shows no known observations of parental care and no large upstream spawning migrations, with eggs and sperm released simultaneously over the spawning substrate.
Spawning runs on male display and peaks in the morning, as in rainbowfishes generally. A receptive female follows a displaying male into fine-leaved vegetation, and the pair release a few eggs at a time rather than one large clutch. Being a shoaling fish, it spawns most readily when kept and conditioned in a group rather than as an isolated pair.
Breeding
M. parva breeds in captivity along the standard rainbowfish lines, and that breedability has been central to its survival. Hobby accounts describe it as an egg-scatterer that spawns readily on acrylic-yarn mops: a conditioned pair deposits eggs of about 0.5 in bearing a few sticky filaments that anchor them to the mop, and once a pair has spawned the adults show no further interest in the eggs — there is no care, and they will eat eggs and fry if given the chance.
The practical method used by breeders is to move the spawning mop, with its adhering eggs, to a separate rearing container — often a pond with natural plankton — so that the fry hatch and grow away from the adults. The newly hatched fry are tiny and hang near the surface, requiring very small first foods such as infusoria, rotifers, or commercial liquid fry food before graduating to newly hatched brine shrimp. Aquarium observation puts first maturity at around 1 in SL and roughly six months, so generations turn over quickly under good conditions — one reason it could be propagated for the trade. By this route M. parva reached commercial breeders in Indonesia, Europe, and North America.
In the aquarium
A peaceful, active shoaling rainbowfish, M. parva should be kept in a group — guidance suggests at least five fish, in a tank on the order of 40 US gal given how much it swims. It wants open water framed by planted margins, and like other rainbowfishes it colours up and behaves best in numbers rather than ones and twos.
Aim for roughly neutral water, around pH 7.0, warm and stable. The fish is hardy and forgiving of a reasonable range, so consistency and good filtration matter more than chasing exact numbers. Tankmates should be similarly sized, peaceful, and not so timid that they get outcompeted at feeding; other rainbowfishes and quiet community fish suit it well. Most stock in the hobby is tank-bred, which makes sourcing captive-bred fish both easy and, for a Critically Endangered species, the right call.
Conservation
Melanotaenia parva is assessed as Critically Endangered (CR), possibly extinct in the wild, on the IUCN Red List (assessment dated 4 December 2019, published 2020). The category is driven by its extraordinarily restricted range — a single lake of roughly 500 by 800 metres and its outlet stream — combined with severe degradation of that one habitat. Once very abundant along the vegetated shoreline of Lake Kurumoi, the species has been pushed to the brink by an infestation of introduced tilapia, which has altered the lake's vegetation and sediment and contributed to a drop in water level that has fallen below the lake's natural outlet.
A fish confined to one small lake has no outlying population to buffer such a collapse, which is why the assessors flag it as possibly extinct in the wild. The aquarium hobby is the saving grace: M. parva was collected and bred before its decline and reached commercial breeders in Indonesia, Europe, and North America, so a captive population persists even as the wild one may have vanished. Here hobbyist breeding carries genuine conservation weight — keeping and propagating captive-bred stock holds a lineage that the wild may no longer have.