Freshwater Fish

Boeseman's Rainbowfish

Melanotaenia boesemani

Half blue, half orange, all motion · Intermediate

Overview

Boeseman's rainbowfish is instantly recognisable among aquarium fish for its split colouration, with a smoky blue-grey head and forebody giving way to a saturated orange-red rear half.
Adult males are deep-bodied, fast and constantly on display, and a shoal of them in morning light is one of the most striking sights in freshwater keeping.
It is the first rainbowfish in this guide, and unlike the soft-water tetras and rasboras it prefers harder, slightly alkaline water.

The species is endemic to the Ayamaru Lakes region of West Papua, Indonesia, and is classed as Endangered in the wild after decades of over-collection and habitat change.
Virtually every fish in the trade is farm-bred, and it should stay that way, so buy captive-bred stock from a reputable source without exception.

The classic mistake happens in the shop, because juveniles are drab silver and only develop full colour over one to two years as they mature.
Keepers who buy on adult photographs often give up on their 'grey' young fish long before the transformation, so patience is part of the care requirement here.

Keep a group of six to eight or more in a tank at least 120cm (4ft) long and 200 litres (55 gallons), because these are fast, powerful open-water swimmers and tank length matters more than volume.
A 60-90cm tank that suits the tetras and barbs elsewhere in this guide is genuinely too small for adult Boeseman's.
Fit a tight lid, as startled rainbowfish are confident jumpers.

Aim for 25-30C (77-86F), pH 7.0-8.0 and moderately hard to hard water, echoing their warm alkaline lake habitat rather than the tannin-stained blackwater many community fish prefer.
Plant the back and sides densely but leave a clear swimming lane along the full length of the tank.

Dark substrate, a dark background and some early-day light bring out the two-tone colour dramatically.
Keep nitrate low with weekly partial water changes, because rainbowfish fade and sicken quickly in old, acidic, poorly maintained water.

Boeseman's rainbowfish are enthusiastic omnivores with a catch, because despite their size their mouth and throat are small, so large pellets are ignored or spat out.
Feed small floating or slow-sinking pellets and flakes sized for much smaller fish, and note that their upturned mouths mean food on the bottom mostly goes to waste.

Rotate the staple with frozen or live brine shrimp, daphnia and chopped bloodworm for conditioning, and include a spirulina or vegetable-based food regularly, since plant matter is a real part of their natural diet and the carotenoids in it support the orange colour.

Feed once or twice daily, only what is cleared within a minute or two.
These are greedy, fast feeders that run fat easily in captivity, so keep portions small and consider one fast day a week for adults.

This is a robust species when its water is right, and most problems trace back to keeping it long-term in soft, acidic or nitrate-heavy water, which shows as faded colour, clamped fins and susceptibility to skin and fungal infections.
Columnaris and ich are the usual acute complaints and both respond best to early treatment and corrected conditions.

Stock quality is the other big issue, because decades of mass farm breeding have produced lines with weak colour, stunted size and bent spines.
Inspect fish before buying, reject any batch with deformed individuals, and expect to pay more for well-bred stock, which is money well spent over a 5-8 year lifespan.

Quarantine new arrivals for two to four weeks, as mycobacteriosis (fish tuberculosis, a chronic wasting infection) persists in some commercial rainbowfish lines and has no practical cure.
A wasting, hollow-bellied fish that eats but thins should be isolated promptly.

Boeseman's are peaceful but decidedly boisterous, spending the day in fast cruising runs and flashy male sparring matches of flared fins and intensified colour that almost never cause real injury.
Keep both sexes, ideally with females equalling or outnumbering males, because males show their best colour when they have rivals and an audience.

In too small a group or a cramped tank the sparring turns into genuine bullying of the weakest male, which is a husbandry failure rather than a character flaw.
Six is the working minimum and eight or more spreads the energy properly.

Good tankmates are other medium, robust, hard-water-tolerant fish: other rainbowfish species first of all, then larger tetras such as Congo tetras, peaceful barbs, Corydoras and other sturdy bottom dwellers.
Avoid tiny or timid species like ember tetras and celestial pearl danios, which are outcompeted at every feeding, and slow long-finned fish such as pearl gouramis or fancy guppies that find the constant motion stressful.