Boesemani rainbowfish

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: plant safe.

Feeding

An omnivore that takes anything offered: quality flake, small pellets, frozen daphnia and brine shrimp, and live food when available. Colour intensifies on a varied diet with plenty of frozen and live items rotated in alongside a dry staple. Two feedings a day works well; the fish is active enough that overfeeding is not normally a risk in a school. Spirulina-based and astaxanthin-enhanced foods help bring out the orange of the rear half.

Compatibility

• The most popular rainbowfish in the trade. Mature males show a two-tone pattern, blue-grey front and orange rear, that is genuinely striking. Juveniles in the store do not look like this and take time to colour up
• Peaceful but active and fast. Best paired with similarly-paced mid-water species. Discus, angelfish, and other slow feeders get out-competed at meal times
• Prefers hard alkaline water (around pH 7.5-8.0, GH 8 and above), which is unusual among popular tropical community fish. Good companions are livebearers and other hard-water-tolerant species; a soft-acid community tank is the wrong setting
• Males display constantly to each other and a group of several males in a long tank shows much better colour than a single male in a peaceful community. An all-male group is fine
• Jumpers; a tight cover is essential. This is a routine cause of unexpected fish loss in new setups
• Do not mix with other Melanotaenia species. Hybrids are fertile and dilute genetic lines, which matters more here than usual because the wild stock is endangered. Keep separate Ayamaru and Uter (or 'Aytinjo') lines as well if you are working with named-population fish

Origin and habitat

Endemic to West Papua, Indonesia, with two known wild populations: Ayamaru Lake in the centre of the Vogelkop ('Bird's Head') Peninsula, and the smaller Lake Uter about 30 km away across difficult karst country. Older accounts also report fish from nearby Lake Aitinjo (or Aytinjo), which sits adjacent to Uter and may have been collected from the same population. Allen and Cross described the species in 1980, naming it after Marinus Boeseman, the former curator of fishes at the Rijksmuseum in Leiden who collected type material in the 1950s. Heiko Bleher introduced the fish to the aquarium trade in 1983, the same year Indonesian farms started producing it commercially. By the late 1980s the wild population was under serious pressure: roughly 60,000 males were exported per month from the Ayamaru region in 1989, and estimates put the total annual collection close to a million fish. Indonesian government restrictions now limit collection. The IUCN currently lists the species as Endangered. Captive-bred stock now dominates the trade. The two wild populations are visibly different and are genetically differentiated as well: Ayamaru fish have a blue-grey front and a vivid yellow-orange rear, Uter fish have a blue front with deeper red on the rear. A microsatellite study (Nugraha et al. 2015, Tropical Conservation Science) found dozens of private alleles distinguishing the two lakes (48 in Ayamaru, 23 in Uter), confirming that they should be kept separate in any captive-breeding programme. The same study showed that all six Jakarta-area farms it sampled were stocked entirely from the Ayamaru population, with no measurable deficit in heterozygosity, which weakens (without disproving) the common farm explanation that declining colour, growth, and fecundity in cultured stock is straightforward inbreeding depression. Lake Ayamaru itself is clear, alkaline (pH around 8.0), about 10 m at maximum depth and around 55 m above sea level, with dense aquatic vegetation. The body is oblong, laterally compressed, with the diagnostic two-tone colour pattern in mature males: a blue-grey or steel-blue front, an orange (or in some lines red) rear half, narrow dark bars, and a white outer margin on both dorsal fins. Females stay a duller silvery colour. Males reach about 11 to 11.5 cm and stop growing there; females are smaller. Sexually active males flash a white spot on top of the head during display.

Breeding

Spawns readily once mature and is one of the easier rainbowfish to produce at home. Unlike many egg-scattering tetras, female rainbowfish drop only a few eggs at a time and continue spawning daily over an extended period, sometimes weeks, when conditions and feeding are right. Spawning happens in the early morning; males flash a white patch on top of the head as part of the display. Eggs are adhesive and get deposited on fine-leaved plants or a spawning mop. Female clutch is small: roughly ten to twenty eggs per morning, hatching after about a week to ten days at typical temperatures (some sources say closer to two weeks). Fry are very small and need infusoria or a fine commercial liquid food before they can take baby brine shrimp. Adults eat their own eggs; mops or plants need to be moved to a separate rearing tank, or the adults removed. Because the wild population is endangered and the cultured population may be losing vigour, do not crossbreed with other Melanotaenia species; the offspring are fertile and dilute genetic lines that may turn out to be conservation-important.

Common problems

Washed-out juvenile colour is the most common surprise. Juveniles look like dull silvery fish and bear very little resemblance to the brightly-coloured adults pictured on retailer photos; a Boesemani needs to reach near-adult size, a settled tank, and ideally a group with other males to compete against before it shows the full blue-and-orange pattern. The species is also unusually susceptible to Mycobacterium infection (sometimes called 'fish TB' or wasting disease in rainbowfish), which presents as a slow loss of weight in fish that are still eating; there is no reliable cure, so affected fish should be removed and equipment sterilised before adding new stock. Quarantining new fish for several weeks before adding them to an established group is standard practice. Jumping is the other practical issue: the species is active near the surface and a tight-fitting lid is essential. Indonesian farm-reared stock has been reported to show declining colour, growth rate, and fecundity across generations, though as noted the microsatellite data does not strongly support inbreeding as the cause.

Bioload

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size