Boeseman's Rainbowfish

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.

Housing

• 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.

Diet

• 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.

Health

• 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.

Temperament

• 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.

Is your enclosure big enough?

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Is this urgent?

In a fish tank the patient is usually the water. Test before you treat — dosing a healthy tank for a disease it does not have makes things worse.

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Never add a new fish to a tank that is not fully cycled, and never treat blind. A test kit is cheaper than the fish.

This is general guidance for a healthy adult, not a diagnosis. An exotics vet who knows your animal always overrides it — and the time to find one is before you need one.

What it really costs

Setup, monthly, first year and across a whole life, itemised and with the electricity worked out from the actual wattage. Published figures for the same animal disagree by a factor of four, so the calculator shows ranges rather than picking one.

Work out the cost of a boeseman's rainbowfish