Boeseman's Rainbowfish (Melanotaenia boesemani)

Melanotaenia boesemani, Boeseman's Rainbowfish, is arguably the most visually dramatic member of the rainbowfish family and one of the most beautiful freshwater fish in the aquarium hobby. Named after Dutch ichthyologist Marinus Boeseman, this species is endemic to just a few interconnected lakes in the Ayamaru region of West Papua (formerly Irian Jaya), Indonesia — making it one of the most geographically restricted rainbowfish species.

The bicolor pattern that makes this species instantly recognisable is striking and unique: the front half of the body is a deep metallic blue-purple to violet, while the rear half transitions sharply into a brilliant orange to yellow-orange — creating a two-tone division of colour unlike almost any other aquarium fish. Males in prime condition are extraordinary, especially under appropriate lighting that brings out the iridescent qualities of both halves.

Females are less dramatically coloured, displaying a more muted version of the bicolor pattern, and are appreciably slimmer than the deep-bodied males. The sexual dimorphism is moderate but clear — males also have a higher body depth and more vivid colours.

Boeseman's Rainbowfish is listed as Endangered on the IUCN Red List due to habitat loss and over-collection in its extremely limited native range. However, it is extensively captive-bred worldwide and all aquarium specimens are captive-produced. Keeping and breeding this species in captivity contributes to its long-term aquarium preservation.

Tank Setup: A minimum 120-litre aquarium for a school of eight (larger is strongly preferred). Provide open mid-water swimming space with planted edges and background. Good surface agitation for oxygenation. A tightly fitting lid — rainbowfish jump readily. Moderate to strong filtration.

Water Parameters: Temperature 24–28°C; pH 7.0–8.0; GH 8–20 °dH; KH 5–10 °dH. Prefers hard, slightly alkaline water — Belgian tap water is generally ideal. Avoid very soft or acidic conditions.

Feeding: Omnivore. Feed quality floating flake, micro pellets, frozen bloodworm, brine shrimp, and Daphnia. Feed two to three times daily. Varied diet enhances colour vibrancy in males.

Tank Mates: Excellent community fish with medium-sized peaceful species: other rainbowfish, medium tetras, barbs, corydoras, and loaches. Best displayed in a school of their own species for the full colour impact. Avoid slow, long-finned species.

Behaviour: Active, energetic schooling fish. Males compete with vivid displays — this colour intensification is normal and non-harmful. Groups of 8+ with more females than males show the most dynamic schooling behaviour.

Health: Hardy once established. Nitrate sensitive — maintain below 20 ppm. Weekly 25% water changes are essential. Prone to fin rot if water quality deteriorates. Avoid overheating above 30°C.