Boeseman's rainbowfish

Boeseman's rainbowfish is one of the most visually spectacular freshwater fish in the hobby, native to just three lakes — Ajamaru, Hain, and Aitinjo — in the remote Bird's Head Peninsula of West Papua, Indonesia. The species exhibits dramatic sexual dichromatism: the front half of the body is a deep metallic blue-grey or violet, transitioning sharply to a fiery orange or yellow on the posterior half. Males reach about 11 cm and develop a distinctly deeper, more compressed body than females, which are smaller and silverer overall. Listed as Endangered on the IUCN Red List since 2020, the wild populations face pressure from habitat degradation, making well-managed captive populations important.

Boesemani rainbowfish are active, peaceful shoalers that do best in groups of six or more. In a group, males constantly display to one another, intensifying their already vivid colours in a shimmering competition that is a joy to watch. They occupy the middle and upper water levels and are excellent dither fish, encouraging shyer tankmates to emerge. Suitable companions include other rainbowfish of similar size, larger tetras, peaceful barbs, and Corydoras catfish. Avoid slow-moving or long-finned species that may be nipped by their constant activity. They are omnivorous, accepting quality flake and granules as a staple, but regular offerings of live and frozen foods — brine shrimp, mosquito larvae, and daphnia — are essential for maintaining their intense colours and breeding condition.

Despite their restricted natural range, Boesemani's rainbowfish adapt well to captivity when their basic needs are met. A tank of 110 litres (about 30 US gallons) is the minimum for a school, though larger is better — they are powerful, active swimmers. The water must be hard and alkaline: pH 7.0 to 8.0, hardness 10 to 20 dH, and a steady temperature of 27 to 30°C. They are strictly tropical and will not thrive in cooler or soft, acidic water. A dark substrate, bright lighting, and open swimming areas bordered by hardy plants suit them best. Boesemani's are egg-scatterers that spawn over fine-leaved plants or spawning mops over several days; the eggs hatch in 7 to 12 days. Aquarium lifespan estimates are around 6 years with suitable care.