Lake Kutubu rainbowfish
Melanotaenia lacustris
The turquoise rainbowfish: mature males' bodies are an intense blue-turquoise with cobalt reflections fading toward the belly in lighter tones, with orange-red fins — a chromatic combination fit for a museum aquarium. Endemic to Lake Kutubu in Papua New Guinea, it is considered vulnerable in the wild — nearly all specimens in the trade are captive-bred. Like M. herbertaxelrodi, it prefers slightly alkaline, hard water (pH 7.0–8.5), not acidic. Males in breeding condition show an orange stripe on the forehead during courtship.
Water Temperature
21 - 26 °C
pH Value
7 - 8
Water type
Freshwater
Tank level
Middle
Adult size
10 cm
Minimum tank volume
>= 110 L
Minimum group
>= 6
GH
7 - 14 dGH
KH
3 - 7 dKH
TDS
135 - 280 ppm
Conductivity
211 - 438 uS/cm
Species description
Geographical Origin and Habitat: Endemic exclusively to Lake Kutubu and its immediate tributary streams in the isolated Southern Highlands Province of Papua New Guinea. Melanotaenia lacustris (the Lake Kutubu Rainbowfish, or Turquoise Rainbowfish) is a highly localized species restricted to this single, pristine mountain lake. It naturally colonizes the shallow, intensely sunlit margins of the lake, thriving in crystal-clear water among incredibly dense, tangled mats of submerged aquatic vegetation, fallen logs, and rocky outcrops.
Taxonomy and Morphology: Scientifically classified within the Melanotaeniidae family, it is one of the most famous and universally recognizable rainbowfish due to its unique coloration. Taxonomically, its specific name "lacustris" translates directly from Latin as "pertaining to a lake," highlighting its exclusive lacustrine (lake-dwelling) origin. Morphologically, mature adults possess a deeply keeled, broad, and heavily laterally compressed body, creating a striking, almost circular diamond shape adapted for powerful swimming.
Social Behavior: They are peaceful, highly active, and strictly obligate schooling fish. To feel secure and display their complex social hierarchy, they must be kept in a massive group (absolute minimum 6-8, ideally 10+). They continuously patrol the middle and upper levels of the tank at high speeds. Early mornings are defined by spectacular, harmless sparring displays among dominant males, who violently flare their fins and "flash" their colors to intimidate rivals and court females.
Coloration and Sexual Dimorphism: Sexual dimorphism is absolute and stunning. Females and juveniles exhibit a muted, highly camouflaged silvery-olive base coloration with largely transparent fins. Males undergo a spectacular transformation upon maturity. The male’s entire body blazes with an intensely glowing, iridescent turquoise, cobalt, or deep metallic blue. Depending on their mood and dominance, this blue can instantly shift to feature deep green hues or a stark, contrasting dark vertical stripe across the head.
Care, breeding and tankmates
Aquarium Setup: The aquarium architecture must perfectly balance massive swimming space with dense, heavy structure. A long tank (minimum 120cm / 4 feet) is strictly required to accommodate their explosive, high-speed swimming bursts. The layout should replicate a densely overgrown lake margin, featuring robust driftwood, smooth stones, and extremely dense thickets of tall background plants (like Vallisneria or Amazon Swords). The entire central and upper water column MUST remain completely open.
Diet and Feeding: In their natural lake margins, they are voracious, opportunistic omnivores, feeding heavily on terrestrial insects that fall onto the surface, aquatic larvae, crustaceans, and significant amounts of filamentous algae. In captivity, they are ravenous, aggressive surface and mid-water feeders. They require a high-quality, vegetable-rich flake or pellet as a staple. This MUST be heavily supplemented with live or frozen foods (bloodworms, daphnia, brine shrimp) to trigger the male’s brilliant turquoise colors.
Water Quality: Originating from an isolated, pristine highland lake, they demand excellent water quality and high oxygen saturation. They prefer slightly cooler tropical temperatures (22-26°C) and thrive in slightly alkaline, moderately hard water (pH 7.4 - 8.5). They require strong, consistent water flow and vigorous surface agitation to ensure maximum oxygenation, simulating the wind-swept currents of Lake Kutubu. They have absolutely zero tolerance for elevated nitrates or ammonia.
Compatibility and Cohabitation: They are entirely peaceful but intensely hyperactive. Their massive, deep-bodied size (reaching 10-12cm) and constant, high-speed swimming will severely stress timid, slow-moving fish (like Discus or Angelfish). Excellent tankmates include other similarly sized, robust schooling fish (like Congo Tetras, large Danios, or other Rainbowfish) and active bottom dwellers like large Corydoras, Hillstream Loaches, or Botia loaches. They are completely safe with robust plants.
Aquarium Breeding: Breeding is relatively straightforward for dedicated aquarists. They are continuous, scatter-spawning fish. A well-conditioned pair will spawn daily at dawn, usually among dense vegetation or artificial woolen spawning mops. The female scatters sticky eggs, which the male fertilizes instantly. The adults will aggressively eat the eggs, so the spawning mop must be removed to a separate, well-aerated hatching tank. The fry are microscopic, stay near the surface, and require infusoria immediately.
Risks and Diseases: Physically, they are incredibly robust, hardy, and highly disease-resistant if provided with adequate swimming space and pristine, hard water. The absolute greatest risk is keeping them in cramped tanks (under 100cm), which inevitably leads to severe psychological stress, stunted growth, and sudden death from crashing into the glass. Medically, they are highly sensitive to soft, acidic water, which will quickly cause their spectacular turquoise colors to wash out completely.
Geographic Origin and Distribution
Endemic exclusively to Lake Kutubu and its immediate tributary streams in the isolated Southern Highlands Province of Papua New Guinea. Melanotaenia lacustris (the Lake Kutubu Rainbowfish, or Turquoise Rainbowfish) is a highly localized species restricted to this single, pristine mountain lake. It naturally colonizes the shallow, intensely sunlit margins of the lake, thriving in crystal-clear water among incredibly dense, tangled mats of submerged aquatic vegetation, fallen logs, and rocky outcrops.
Origin
Africa and MadagascarAustralia, New Guinea, and Oceania
Fish profile
- Temperament
- Peaceful
- Their temperament is generally peaceful. Males frequently spar playfully without causing harm. They add dynamic energy to community tanks.
- Diet
- Omnivore
- They are naturally omnivorous. They readily accept flakes and small frozen foods. A varied diet enhances their vibrant colors.
- Life expectancy
- 5-8 years
- Lake Kutubu Rainbowfish typically have a lifespan of 5 to 8 years under optimal captive conditions. To help them reach this age, it is crucial to maintain pristine water quality, high dissolved oxygen levels, and a stable, slightly alkaline pH. A balanced diet consisting of high-quality foods and a spacious tank for continuous swimming will also support their long-term health. Minimizing stress by avoiding aggressive tankmates and keeping them in a school is essential for their longevity.
- Adult size
- 10 cm
- The catalog's scalar value is 10 cm. The FishBase SPECIES table (v25.04 snapshot) source reports a maximum adult measurement of 10 cm for this species, recorded as standard length (SL). The taxonomic record consulted is Melanotaenia lacustris (v25.04); male or unsexed record. These figures are not interchangeable: the first is the profile's editorial reference, whereas the second is a published maximum under a specific measurement convention; differences among sources can reflect sex, population, life stage and measuring method.
- Sex ratio
- Harem
- Keep one male to multiple females. This significantly reduces male harassment. Multiple males can coexist in larger aquariums.
- Feeding frequency
- Twice daily
- Feed small portions twice a day. They consume food rapidly from the surface. Avoid overfeeding to maintain water quality.
- Bioload
- Medium
- They produce a moderate bioload. Efficient filtration is needed for their active lifestyle. Regular water changes maintain pristine conditions.
- Flow
- Strong
- They appreciate moderate to strong flow. This simulates their native river habitats. Good circulation is highly beneficial.
- Jump risk
- Yes
- There is a very high jump risk. A tight-fitting lid is absolutely essential. Many are lost to open tanks.
- Reproduction
- Oviparous
- They are prolific egg scatterers. Parents will eat eggs if not removed. Fry require extremely small initial foods.
- Compatibility & tankmates
- Community
- They are peaceful but highly active. Avoid housing with slow or timid species. They thrive with other energetic fish.
- Tank level
- Middle
- They swim in the middle and upper levels. They require plenty of open swimming space. Perimeter plants provide security.
Scientific Parameters & Models
FishBase
Window to the official scientific data of the species
Data synced on: 06/15/2026
- Trophic level
- 2.9 ± 0.4 se
- Resilience
- Medium (1.4 - 4.4 years)
- Phylogenetic uniqueness (PD50)
- 0.5
- Bayesian Length-Weight
- a = 0.01122, b = 3.04
- VulnerabilityLow vulnerability (10 / 100)
Image gallery
Licensed images linked to the species or, when marked, to the closest representative taxon.
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