Boesemani Rainbowfish (Melanotaenia boesemani)

Colour Forms & Morphs

Boesemani Rainbowfish are among the most dramatic examples of delayed colour development in the hobby. Juveniles sold at 3–4 cm look disappointingly plain — pale silver with a faint yellowish tinge. Many first-time buyers wonder if they received the wrong fish. Rest assured: the transformation is coming. Males begin showing the blue-orange split around 5–6 cm, and colour continues to deepen and intensify for the first two to three years of life. Diet plays a pivotal role — regular offerings of carotenoid-rich foods such as spirulina flake, astaxanthin-enriched pellets, daphnia, and brine shrimp will accelerate and amplify the orange tones. Morning light hitting a school of mature males in full display is genuinely one of the great spectacles in freshwater fishkeeping. Males will also ‘flash’ — briefly intensifying their colour during sparring or courtship — creating pulses of neon brilliance that no photograph can fully capture.

Telling Males from Females

Sexual dimorphism in Boesemani Rainbowfish is among the most pronounced in the Melanotaeniidae family, but only once the fish reach maturity. Below 5 cm, sexing is essentially guesswork. As males mature, they develop a characteristic deep body profile and, in older specimens, a noticeable nuchal hump — a raised forehead that becomes quite prominent in dominant males over 3 years old. The dorsal and anal fins grow proportionally larger and more pointed in males, and the first dorsal ray may extend into a short filament.

Colour is the most reliable indicator in adult fish. Males display the full bicolour pattern with saturated blue and orange zones, while females remain olive to silver with muted golden tones. During morning display behaviour, males will line up alongside rivals and competitors, flaring fins and intensifying their colours in rapid pulses — a behaviour known as ‘jousting’ or ‘flashing’. These displays are harmless and are one of the most captivating daily behaviours in a rainbowfish aquarium.

For the best colour displays and most natural behaviour, aim for a ratio of roughly equal males and females, or a slight male majority. A group of at least six — ideally eight to twelve — gives the males enough social stimulus to display without any single individual being constantly harassed.

Where the Name Comes From

The species was formally described by Gerald Allen and Norbert Cross in 1980 and named in honour of Marinus Boeseman, a distinguished Dutch ichthyologist at the Leiden Museum of Natural History. Boeseman spent decades cataloguing the freshwater fishes of New Guinea, and this spectacular rainbowfish was one of his most celebrated discoveries — a fitting tribute to a scientist whose field work opened western eyes to the astonishing biodiversity of Papuan waterways.

The genus name *Melanotaenia* translates roughly as ‘black-banded’ (from the Greek *melano-* meaning dark and *taenia* meaning ribbon or band), referencing the subtle dark lateral stripe found in many rainbowfish species. In the Boesemani, this stripe is largely masked by the dramatic bicolour pattern, but it can still be glimpsed in juveniles and subdominant individuals.

In the hobby, the fish is universally known by its scientific epithet — simply ‘the Boesemani’. It arrived in the aquarium trade in the early 1980s and caused an immediate sensation. The striking colour split — cool blue at the head transitioning to warm orange at the tail — was unlike anything hobbyists had seen in a freshwater fish, and demand quickly outstripped supply. Fortunately, the species breeds readily in captivity, and virtually all specimens available today are captive-bred, relieving pressure on the endangered wild populations in the Ayamaru Lakes system.

Feeding Guide

Boesemani Rainbowfish are enthusiastic, unfussy omnivores that will eat virtually anything offered. In the wild, they feed on a mix of small invertebrates, algae, insect larvae, and plant matter. In captivity, a varied diet is the key to vibrant colour and robust health.

The staple diet should be a high-quality flake or micro-pellet formulated for tropical omnivores. Look for products containing spirulina and astaxanthin — these carotenoid pigments are directly responsible for intensifying the orange and red tones. Feed two to three small meals per day rather than one large feed; this matches their natural grazing behaviour and reduces waste.

Supplement the staple with frozen foods two to three times per week: bloodworms, brine shrimp, daphnia, and mysis shrimp are all eagerly accepted. Live foods such as mosquito larvae, wingless fruit flies, or cultured daphnia are excellent conditioning foods, especially when preparing fish for breeding.

Vegetable matter should not be overlooked. Blanched zucchini medallions, spirulina wafers, or algae-based flakes ensure the fish receive the plant-based nutrition they consume in the wild. A balanced omnivorous diet — not just protein-heavy frozen foods — produces the best long-term colour, body condition, and lifespan.

Breeding Guide

Day 0

Conditioning

Increase live and frozen foods for 1–2 weeks; raise temperature to 28–29 °C

Day 7–14

Spawning

Males display at dawn; eggs scattered on fine-leaved plants or spawning mops

Day 14–21

Egg Collection & Incubation

Remove mops to a separate hatching tank; eggs hatch in 7–10 days at 27–28 °C

Day 21–28

Fry Emerge

Tiny fry absorb yolk sac; begin feeding infusoria and vinegar eels

Day 28–60

Growing On

Graduate to baby brine shrimp and micro-worms; slow but steady growth

Month 3–12

Juvenile Development

Gradual colour development; full adult colour at 12–18 months

Conditioning

Successful Boesemani breeding begins with thorough conditioning. Separate males and females for one to two weeks if possible, or simply increase the proportion of live and frozen foods in the diet — daphnia, brine shrimp, and bloodworms are ideal. Gradually raise the water temperature to 28–29 °C and ensure water quality is pristine with nitrate below 10 ppm. Well-conditioned females will appear noticeably rounder when viewed from above.

Spawning

Boesemani Rainbowfish are egg scatterers that spawn at first light. Males will intensify their colours dramatically and perform vigorous parallel displays alongside females, quivering and flashing. When a female is receptive, the pair will move side by side into fine-leaved plants or synthetic spawning mops, releasing and fertilising small batches of adhesive eggs. A single female may scatter 10–30 eggs per session over several consecutive mornings. The eggs are tiny (approximately 1 mm), clear, and have adhesive filaments that attach them to plant surfaces.

Egg Collection & Incubation

Adults will readily eat their own eggs and fry, so spawning mops or plant bunches should be removed to a separate hatching container every two to three days. A small 20–40 litre tank with gentle aeration and matching water parameters works well. Add a few drops of methylene blue to prevent fungus on unfertilised eggs. At 27–28 °C, healthy eggs will hatch in approximately 7–10 days. The incubation period is notably longer than many other tropical species, so patience is required.

Fry Emerge

Newly hatched fry are extremely small — among the tiniest of commonly bred aquarium fish. They will hover near the surface for the first day or two, absorbing the remainder of their yolk sac. First foods must be correspondingly tiny: infusoria, paramecium cultures, or vinegar eels are ideal. Commercial liquid fry food can supplement but should not replace live micro-organisms in the first week. Keep the water level low (10–15 cm) to make food easier for the fry to find.

Growing On

After two to three weeks, fry can be transitioned to freshly hatched baby brine shrimp (Artemia nauplii) and micro-worms. Growth is notably slow compared to livebearers or cichlids — expect the fry to reach only 1–2 cm after two months. Frequent small water changes (10% daily or every other day), warm stable temperatures, and multiple small feeds per day are essential for healthy development. Fry mortality is highest in the first two weeks; after that, survival rates improve significantly.

Juvenile Development

Boesemani Rainbowfish are famously slow to colour up. Juvenile fish remain plain silver-grey for months, with only subtle hints of the adult pattern beginning to appear around the 4–5 cm mark. Males will start showing a faint blue wash on the anterior and golden tones on the posterior by around six months. The full, spectacular bicolour pattern typically does not develop until 12–18 months of age. This slow maturation is normal and should not cause concern — the wait is absolutely worth it.

Compatible Species

Boesemani Rainbowfish are peaceful community fish in the truest sense — they have no interest in territory, do not nip fins, and ignore species that don’t compete directly for their food. However, they are large, fast, and boisterous. Their energetic swimming and enthusiastic surface feeding can inadvertently intimidate slow-moving, timid, or very small tank mates.

The ideal community pairs Boesemani with similarly sized, active fish that occupy complementary water column zones. Other rainbowfish species make the most natural companions — a mixed Melanotaeniid community tank is a truly spectacular display. Robust tetras, barbs, corydoras, and loaches all work well. Avoid pairing with anything that requires calm, still water or cannot compete at feeding time.

A school of at least six Boesemani (ideally eight or more) is essential. Lone individuals or pairs become shy, dull in colour, and prone to stress. In a properly sized group, males will spend their energy displaying to each other rather than pestering other species, and the school dynamics create a far more natural and visually engaging aquarium.