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Care guide · Freshwater · fish

Lake Kutubu rainbowfish

Melanotaenia lacustris

Lake Kutubu rainbowfish (Melanotaenia lacustris)
Photo: Thomas Gräfe (Thomnight), CC BY-SA 2.0 DE. Photo: Thomas Gräfe (Thomnight) · Source · CC BY-SA 2.0 GermanyResized and compressed; no cropping or colour changes.

The Lake Kutubu rainbowfish (Melanotaenia lacustris), also called the turquoise rainbowfish, is an active, striking blue-green species from Papua New Guinea. Juveniles are often much duller than adults, and the specialist care profile notes that full colour may take more than a year to develop. This fish needs a long aquarium and a group of its own kind. Suitable water readings do not make a short, cramped tank appropriate: swimming length and adult size matter. Its natural range is restricted to Lake Kutubu and the outlet Soro River. The more recent IUCN assessment lists it as Endangered, so ask suppliers for verifiable captive-bred provenance.

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At a glance

Temperature
20–25 °C
pH
7.0–9.0
GH
10–20 °dGH
Aquarium length
At least 120 cm
Minimum aquarium volume
At least 108 L
Adult size
Up to 12 cm
Group size
At least 6 individuals
Temperament
Peaceful

About this species

The accepted scientific name is Melanotaenia lacustris Munro, 1964. GBIF and the Australian government assessment place it in Melanotaeniidae and the order Atheriniformes. Body colour ranges from steel blue to turquoise; mature males are generally larger, deeper-bodied and brighter than females. The government assessment gives a maximum near 12 cm, while many fish remain under 10 cm. Seriously Fish lists 12 cm standard length. Because sources do not always use the same measurement, treat 12 cm as an upper bound rather than a prediction for every fish. The golden-orange nape can become more conspicuous during display or spawning.

Natural habitat

This rainbowfish is known from Lake Kutubu and the Soro, its outlet river, in the Southern Highlands of Papua New Guinea. The clear, mostly still lake water is mildly alkaline, and fish occur around shallow vegetated margins, roots and submerged wood. This describes its habitat; it is not a requirement to reproduce the lake exactly in an aquarium. Its restricted range and pressures around the lake make provenance relevant. Prefer fish for which the supplier can confirm captive breeding, and never release aquarium fish or eggs into natural waters.

Aquarium and layout

Seriously Fish recommends a footprint of at least 120 × 30 cm and gives approximately 108 litres for a tank 30 cm high. Treat that volume as the geometry of the stated minimum dimensions, not as a guarantee that every stocking plan is responsible at 108 litres. The species is fast and active; a larger, longer aquarium is preferable for a group of six or more and should allow for adult size. Combine dense planting along parts of the back and sides with broad open swimming space. Fit a secure lid because startling and jumping are practical risks with active fish. Avoid unnecessarily strong current and maintain mature biological filtration.

Water conditions

For aquarium care, Seriously Fish gives 20–25 °C, pH 7.0–9.0 and GH 10–20 dGH. These are tolerance ranges, not a reason to swing between extremes. Keep conditions stable, mineral-bearing and not soft-acidic; the specialist source warns that soft, acidic water is unsuitable. FishBase contains an older aquarium reference of 27 °C, but it differs from the current species profile and is not included in the recommended range. The lake’s natural pH is habitat context, not a separate aquarium target. Test your source water and acclimate fish gradually.

Behaviour and group size

Keep Lake Kutubu rainbowfish in a group of at least six; the specialist describes a shoal of six to eight or more as reducing skittishness and supporting normal social behaviour. Plan for the space adult fish need, not only juveniles. The species is not described as aggressive, but its size and rapid swimming can disturb small or quiet fish. The specialist lists broad possible tankmate groups, yet no individually checked combinations have been recorded in this guide’s central relationship network. We therefore do not present any particular species as guaranteed suitable. Assess combinations by adult size, swimming zone, temperament, water and available swimming length.

Feeding

It is an omnivore that, according to Seriously Fish, accepts dried, frozen and live foods readily. Use an appropriate complete dry food as a staple if the fish actually take it, and vary the diet with small live or frozen invertebrates. One existing catalog product is linked as a complete flake option, not as an exclusive diet; dried daphnia is listed only as an occasional supplement. Feed modest portions and check that all fish, including less dominant individuals, get food. Good nutrition does not guarantee rapid colour development: adult colour takes time and varies among fish.

Breeding

This species is an egg scatterer. For breeding, Seriously Fish describes a separate, quiet setup with slightly hard alkaline water, around pH 7.5 and 22–24 °C, plus fine-leaved plants or spawning mops. These are source-specific breeding conditions, not a reason to change the main aquarium abruptly. Eggs may attach to surfaces by a fine thread, and adults often eat them. Check mops or plants and transfer eggs to a separate rearing tank if needed. The specialist gives an approximate hatch time of 7–12 days depending on temperature. Newly hatched fry are tiny and need very fine food first, followed later by brine shrimp nauplii. Raising them therefore requires more planning than spawning itself.

Sources & review

This care guide draws on several specialist sources and was last checked on .

View the sources