Taxonomy & naming
Glossolepis incisa was described by the Dutch zoologist Max Weber in 1907 from material collected in Lake Sentani in what was then Dutch New Guinea. Eschmeyer's Catalog of Fishes records the valid name as Glossolepis incisa Weber, 1907; the masculine spelling 'incisus' is common in the aquarium literature and on FishBase, but the name was published as 'incisa' and the valid name follows the original. Because the species was described in Glossolepis, where it remains, no parenthetical authority is used.
The genus Glossolepis is one of the rainbowfish genera of the family Melanotaeniidae — the true rainbowfishes and their blue-eye relatives in the order Atheriniformes — and is centred on New Guinea. Glossolepis species resemble the larger genus Melanotaenia in body plan, sharing the family's deep, laterally compressed body and divided dorsal fin; the genera are separated on fine morphological characters rather than any single obvious feature. Within the hobby, G. incisa is the best known member of its genus — the 'red rainbow' against which the others are compared.
Meristic counts reported for the species include 6 to 7 dorsal spines and 9 to 10 dorsal soft rays in the second dorsal, a single anal spine, and 18 to 20 anal soft rays, on a fusiform-to-deep body that becomes markedly deeper with age in dominant males.
Morphology
The red rainbowfish is a large, deep-bodied rainbowfish, reaching about 4.5 in standard length in males and around 4 in in females, with the back becoming strongly arched in older, dominant males so that the body takes on the high, almost humpbacked profile characteristic of the species. As in other Melanotaeniidae, the dorsal fin is divided into a short anterior portion and a longer posterior portion, and the body is strongly compressed from side to side.
Colour is the species' defining feature, and it is sharply sexually dimorphic. Mature, dominant males run salmon to brick red over the whole body, often overlaid with scattered silvery or pearly scale spangles that catch the light. Females, subordinate males, and stressed fish are far plainer — a camouflaged brownish to olive with silvery reflections on the head and flanks. Male colour tracks condition and mood: a displaying or dominant male flushes to full intensity, while a recently moved, subordinate, or unwell fish can fade to a dull greyish brown within minutes. That fading is normal for the species, not a sign of poor health on its own.
Habitat
Glossolepis incisa is endemic to Lake Sentani, a large lake at roughly 246 ft elevation at the foot of the Cyclops Mountains in north-eastern Papua (formerly Irian Jaya), Indonesia, near the city of Jayapura. The lake lies at about 3 to 4 degrees south of the equator and drains to the sea through a shallow river, and the species is not naturally known from anywhere else — its entire wild range is this one lake system.
The water is warm, hard, and alkaline. FishBase reports a pH of about 7.6 to 8.2, hardness around 9 to 19 dH, and temperatures of roughly 29 to 30 degrees Celsius for the lake. The fish is a benthopelagic schooling species that occupies densely vegetated, generally clear water subject to seasonal algal blooms; adults shoal among aquatic plants and along vegetated margins. The single-lake endemism is the central fact of its biology. The alkaline, mineral-rich chemistry reflects the local catchment, and there is no outlying wild population to buffer the species against pressures on Lake Sentani itself.
Feeding
Like rainbowfishes generally, the red rainbowfish is an opportunistic omnivore with a reported trophic level of about 2.8, feeding in the middle and upper water column. In Lake Sentani the natural diet comprises small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects taken at or near the surface — together with algae and plant material grazed from submerged vegetation.
In the aquarium it is an easy, enthusiastic feeder that takes good-quality flake and small pellets, frozen or live foods such as Daphnia, brine shrimp, bloodworm, and mosquito larvae, and some vegetable matter. A varied diet with a real live or frozen component improves both colour and breeding condition, and colour-enhancing prepared foods are widely used to hold the red intensity that dulls on a flake-only diet. As a near-constant grazer it does best on modest amounts several times a day rather than one large feeding.
Mating
The red rainbowfish is an egg-scattering plant spawner with no parental care, and under good conditions it spawns continuously rather than in a single seasonal event. Males are social and competitive. A dominant male displays to females with intense colour, flaring his fins and flashing the body so the red peaks, and shows his strongest colour in the presence of rivals. As in many rainbowfishes, a flickering pale courtship band can run along the head and nape of a displaying male, and display is most vivid in the early morning.
Spawning is most active in the first hours after first light. A receptive female follows a displaying male into fine-leaved vegetation or spawning mops, where the pair release small numbers of eggs at a time over the course of a morning. Because males display most strongly against rivals, the species shows its best colour when several are kept together, and breeders typically condition groups rather than isolated pairs to bring fish into peak form. During active spawning males can become pushy toward females, so cover and adequate space help spread out the attention.
Breeding
Glossolepis incisa is one of the more straightforward rainbowfishes to breed, which is much of why it has stayed in the hobby so long. Conditioned adults spawn over fine-leaved plants such as Java moss or over synthetic spawning mops, depositing small adhesive eggs that hang from the medium by fine filaments. Breeders commonly use a single male with two or three females, or condition a larger group; reported daily output is up to roughly 50 eggs per female, released a few at a time over many days rather than in one large clutch. FishBase cites a total fecundity range of roughly 910 to 3,122 eggs.
The water should be kept hard and alkaline to match the species' Lake Sentani origins. At the upper end of the breeding temperature range the adhesive eggs hatch in about six days, the incubation time shortening with warmth. Adults will eat their own eggs and offer no care, so the usual method is to lift the mops or eggs to a separate rearing container. The tiny fry hang near the surface at first and need very small first foods — infusoria, commercial liquid fry food, or egg yolk — before graduating to newly hatched brine shrimp and finely crushed flake. Growth is slow, and males take well over a year to develop full red colour, so patience is part of raising good stock. Resilience is rated medium, with an estimated population doubling time of 1.4 to 4.4 years.
In the aquarium
The red rainbowfish is an active, peaceful, strongly schooling fish that should be kept in a group of at least five or six — more is better, both for the fish's security and because males colour up best when displaying against one another. It is a large, restless rainbowfish and needs real swimming room: a tank of around 39.5 in or more in length suits an adult group, and keeping several males together holds the display behaviour and colour at their peak.
Water chemistry should reflect the species' hard, alkaline origins, not the soft-water default many aquarists reach for with tropical fish. It does best in neutral to distinctly alkaline, moderately hard water; hobby sources suggest a pH around 7.2 to 8.0 and temperatures of roughly 24 to 28 degrees Celsius for general keeping, while the wild lake runs warmer and harder. Stable conditions and good filtration matter more than chasing exact numbers, and the fish tolerates a fairly wide range. A well-planted, well-lit tank with open central swimming space and some surface movement shows the red off best and encourages the morning display. Tankmates should be similarly sized, peaceful, and not so timid that they lose out at feeding — other rainbowfishes, larger tetras, and peaceful barbs and rasboras are natural companions. The species is hardy and has been kept by aquarists for more than forty years; males just need time and good conditions to reach full colour.
Conservation
Glossolepis incisa is assessed as Endangered (EN) on the IUCN Red List under criterion B1ab(iii), in an assessment dated December 2019. The listing follows from the species' single-lake endemism: with its entire wild range confined to Lake Sentani and its feeder waters, the fish has nowhere else to be, and little resilience to local pressures. Documented habitat decline in and around the lake — including degradation of the vegetated shallows the species depends on — drives the status. Older hobby sources that still cite a 'Vulnerable' rating predate the current assessment.
A fish endemic to one lake basin is exposed to introduced predators and competitors, pollution, and shoreline change, none of which a single population absorbs easily. Captive breeding softens the picture: much of the red rainbowfish in the trade today is tank-bred rather than wild-caught, which reduces collecting pressure on Lake Sentani and keeps the species widely established in captivity. Aquarists can help by sourcing captive-bred stock, and the species is a frequent subject of hobbyist and association breeding programmes precisely because of its threatened wild status.