Taxonomy & naming
Melanotaenia exquisita was described by the Australian ichthyologist Gerald R. Allen in 1978, in the magazine Tropical Fish Hobbyist (26(10): 97), with the type locality given as the Edith River, Northern Territory. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia exquisita Allen, 1978; because the species was described in Melanotaenia, where it remains, the authority is cited without parentheses.
Melanotaenia is the largest genus in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia within the order Atheriniformes. M. exquisita is one of the smaller Australian members of the genus, and you tell it apart by colour rather than by meristics — the paired black midlateral stripes, the red stripe beneath them, the zigzag markings toward the anal fin. The fin and scale counts overlap broadly with its congeners: a divided dorsal fin of IV–VI spines followed by I spine and 7–9 rays, an anal fin of I spine and 15–19 rays, and 34–35 vertical scale rows.
Its distribution sits within two disjunct regions: the Edith River Gorge and adjacent headwaters of the Mary and South Alligator River systems in the Northern Territory, and — per Fishes of Australia — the King George River of the Kimberley region in Western Australia. In the Northern Territory it is found alongside the widespread Melanotaenia splendida australis, with which it is sympatric.
Morphology
Melanotaenia exquisita is a small rainbowfish reaching about 3 in standard length, with females smaller — roughly 2 in SL — than males. The body is elongate, slender, and laterally compressed, carrying the family's characteristic two separate dorsal fins, the first originating opposite the anal fin. The jaws bear conical to caniniform teeth in several rows, with additional teeth on the vomer and palatines.
Colour is what sets the species apart. The base is silvery-blue to olive-green, silvery-white below, crossed by a pair of black midlateral stripes. A red stripe runs just below these, with a pair of dark zigzag markings between it and the anal fin. The dorsal, anal, and caudal fins are edged in red and often carry small black spots. That crisp, high-contrast effect is what earned the fish its epithet.
The sexes differ in fin shape and build. Mature males are slenderer and grow a taller first dorsal fin; females have smaller, more rounded dorsal and anal fins. Colour intensity, as in rainbowfishes generally, tracks mood and condition: a displaying or well-conditioned fish flushes far brighter than a stressed or subordinate one.
Habitat
This is a fish of small, clear, swift-flowing streams. It favours steep-gradient watercourses broken by cascades, gathering in the pools at the foot of small waterfalls where the flow is fast, though it also holds in quieter stream and lake sections. It turns up in streams above high waterfalls too, and it prefers reaches fully open to the sun over shaded ones.
In the Northern Territory the species occurs in the Edith River Gorge and Lake Malkyyllumbo within Edith Falls (Leliyn) in Nitmiluk National Park, in the upper South Alligator River system, and at Moline Pool in the Mary River system. The Edith River is a tributary of the Daly River system, lying roughly 143 mi southeast of Darwin. A lake population at the base of Edith Falls shows the species is not tied strictly to running water. Fishes of Australia also records it from the King George River in the Kimberley of Western Australia, in small, clear, rocky stretches near rapids, in rocky pools, or above waterfalls.
The habitat is warm and tropical. FishBase gives a temperature range of about 75–86 °F and places the species between roughly 12° S and 16° S latitude. The constant across localities is clear, well-oxygenated, often turbulent water over rock, fully open to the sun.
Feeding
Melanotaenia exquisita is an omnivore, feeding mainly on aquatic and terrestrial insects and their larvae with green algae on the side. Its reported trophic level of about 2.9 fits a mixed but largely insectivorous diet. Its taste for sunlit, fast-flowing pools at the foot of cascades puts it well placed to pick off drifting invertebrates and terrestrial insects swept into the current, and to graze algae off the rocks.
In the aquarium it is undemanding and takes most foods. Good flake and small pellets make a sound staple; frozen and live foods — Daphnia, brine shrimp, bloodworm, and similar small invertebrates — bring out its best condition and colour. As an active grazer it does best on small amounts several times a day rather than one big feeding, which mirrors the steady drip of drift food in the stream.
Mating
Like rainbowfishes generally, Melanotaenia exquisita forms distinct pairs at spawning, the males courting females by colour display rather than by nest-building or any real territory defence beyond a transient display patch. Details specific to this species are thin on the ground — Fishes of Australia notes that little is known of its reproductive biology — so what follows draws on the genus pattern, in which males deepen their colours and flare their fins to court receptive females through the bright morning hours.
Spawning peaks early, just after first light, when a receptive female follows a displaying male into fine-leaved cover to lay. The eggs are adhesive and hang by fine filaments from the spawning medium. With no data on M. exquisita's own courtship sequence, treat all of this as the genus norm rather than confirmed species-level observation.
Breeding
Detailed breeding accounts for Melanotaenia exquisita are scarce, and published sources call its reproductive biology little known. What is safe to say: it is an egg-scattering, plant-spawning species in the rainbowfish mould, laying small adhesive eggs that hang by fine filaments from fine-leaved plants or synthetic mops, with no parental care.
The genus pattern is well established. Conditioned adults release a few eggs at a time over a long spawning period rather than one big clutch, and they eat their own eggs, so breeders usually move the spawning medium to a separate rearing container. At the warm temperatures the species lives in — the mid-to-upper 20s Celsius — rainbowfish eggs hatch in roughly one to two weeks, and the tiny fry need very small first foods such as infusoria or liquid fry food before they can manage newly hatched brine shrimp. With no firm species-specific figures, take these as genus-typical guidance; data for M. exquisita itself are sparse.
In the aquarium
FishBase rates Melanotaenia exquisita a good aquarium fish, with one caveat: it swims fast and wants well-oxygenated water, so it needs more space and more aeration than its modest size suggests. Give it brisk water movement and high dissolved oxygen — the swift, cascade-fed pools of its range in miniature — and it holds its best health and colour.
Keep it as a group, not singly or in pairs. That suits its shoaling habit and gets the males displaying against one another. Bright lighting echoes the sunlit wild habitat and helps the red-edged fins and stripes show. The 75–86 °F range reported for its habitat is right; stable, clean, well-oxygenated water is the priority.
Small, peaceful, and fond of current, it pairs well with similarly sized, peaceful, current-tolerant tankmates. The whole package — strong swimmer, needs aeration, easy to feed — rewards a keeper who can give it a flowing, well-filtered, brightly lit tank.
Conservation
Melanotaenia exquisita is assessed as Least Concern (LC) on the IUCN Red List, in an assessment dated 15 February 2019 (species ID 13061). This reassessment supersedes earlier listings: the species was assessed as Data Deficient in 1996 (and as Rare in 1994), a Data Deficient status still echoed on some secondary pages, but the current Red List status is Least Concern.
Its security rests on a spread across several stream systems in northern Australia, much of it inside protected areas — Nitmiluk National Park takes in the Edith Falls (Leliyn) localities — and on the remoteness and good condition of its clear-water habitats. A small fish of swift, rocky streams is sensitive to shifts in water clarity and flow, so its chief safeguards are continued protection of the catchments and the clean, well-oxygenated stream conditions it depends on.