Freshwater Fish

Boeseman's Rainbowfish

Melanotaenia boesemani

Half blue, half orange, all motionIntermediate

Lifespan: 5-8 years
Size: Males 4-4.5 in; females to 3 in
Social needs: Shoaler; keep 8+ with both sexes, more is better
Time: Low-moderate (daily feed, weekly water changes)
Cost: Medium (large tank; quality stock costs more)
Diet: Omnivore; small foods for a small throat

Overview

What is this animal?

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 only rainbowfish in this guide, and unlike the soft-water tetras and rasboras it prefers harder, slightly alkaline water.

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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.

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.

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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.
Eight is the working minimum, the top of the six to eight Seriously Fish gives as its least, and more spreads the energy further.

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.

A good fit for

Keepers with a 4 ft tank to fill with colour
Active, hard or alkaline water communities
Patient keepers who can wait 1-2 years for adult colour
A long-lived centrepiece shoal (5-8 years)
Homes wanting constant daytime activity

Common mistakes to avoid

Judging drab juveniles - full colour takes 1-2 years
Tanks under 4 ft cramping this fast swimmer
Soft, acidic or cool water dulling colour and health
Farm stock with bent spines or washed-out colour
Overfeeding greedy fish that easily run fat
An open top - startled rainbowfish jump

Enclosure

Minimum 53 gallons, for a group of 8

Cannot be kept alone. The minimum is 8, and the space above is for that group.

Keep a group of eight or more in a tank at least 4 ft long and 53 US gallons (a standard 55-gallon tank qualifies), because these are fast, powerful open-water swimmers and tank length matters more than volume.
A 24-35 in 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.

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Aim for 77-86°F, 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.

Check your enclosure

Supplies

Setup

Tank and stand
Filter
Heater and thermometer
Substrate, plants and decor
Liquid test kit
Dechlorinator and starter supplies

Ongoing

Small floating or slow-sinking pellets (sized for smaller fish)
Quality tropical flake
Spirulina or vegetable-based food
Frozen brine shrimp and daphnia
Frozen bloodworm (to be chopped)

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Small scissors for chopping frozen food
Fine net for removing leftovers

Safe substrate: Fine, smooth sand · Smooth, rounded fine gravel (not for loaches or sand-sifting cichlids; sand is better for corydoras) · Bare bottom (quarantine and grow-out only)

Work out the cost

Care routine

Daily

Check temperature and equipment
Feed
Head-count and look everyone over

Every few days

Top up evaporationevery 3 days

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Weekly

Check for a thinning fish
Test the water
Water change and gravel vacuum
Wipe the front glass only

Every few weeks

Trim and thin the plantsevery 2 weeks

Monthly

Check hardness as well as nitrate
Check supplies and reagent dates
Replace or recharge chemical media, if you use any (skip it in a blackwater tank)
Rinse the filter media

Health review: Quarterly condition and water-hardness review, every 3 months

Tools

Is your enclosure big enough?Check your measurements against the recommended size What will it cost?Setup, monthly and over its whole life, itemised Is this ammonia reading dangerous?Works out the toxic part from your pH and temperature

Care in depth

Diet

Omnivore; small foods for a small throat

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

Signs of good health, and the common problems

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.

Is this urgent?

6 warning signs and how fast to act, 3 of them right away

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.

Go now

Any detectable ammonia or nitrite in a stocked tankLarge water change now with dechlorinated, temperature-matched water. Stop feeding. Test again in 12 hours. Both are directly toxic. How toxic the ammonia reading is depends on pH and temperature, so the same number can be harmless or lethal.

Now

Fish gasping at the surface with fast gill movement, or crowding the filter outflowIncrease surface movement now (gently in a betta or gourami tank, without creating a strong current), then test the water. Either the oxygen has crashed or the gills are being chemically burned. Both are hours-scale problems. Bettas, gouramis and corydoras rise to gulp air as part of normal breathing; the warning is constant gulping, fast gill movement, or the whole group hanging at the top.

Now

Sudden deaths, more than one fish in a dayTest ammonia, nitrite, nitrate and temperature before doing anything else. Do not add medication yet. Multiple simultaneous deaths usually point at the water or the equipment rather than a pathogen, so rule those out first.

Now

Today

Flashing: repeatedly scraping against decor or substrateTest the water, then look closely for parasites. Irritation is either chemical or parasitic; the test tells you which before you medicate.

Today

White salt-grain spots over the body and finsBegin the appropriate treatment today and check the temperature. The parasite's life cycle is temperature-driven, and it multiplies with every cycle you wait.

Today

Soon

Clamped fins, hiding, or refusing food for more than two daysTest the water and review recent changes. Usually the earliest visible sign of a parameter drifting.

A few days

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.

Bedding and substrate

3 safe, 4 with a caveat, 2 to avoid

Substrate in an aquarium is a filter as much as a floor. Most of the biology lives in and on it.

Safe

Fine, smooth sandCorrect for corydoras, kuhli loaches, eartheaters such as rams and acaras, and anything else that sifts or burrows. Barbels wear away on sharp gravel.

safe

Smooth, rounded fine gravel (not for loaches or sand-sifting cichlids; sand is better for corydoras)Rinse thoroughly before use. Grain size should be too large to swallow and too small to trap food. Kuhli loaches need sand to burrow into and rams and acaras sift mouthfuls of it; bronze and panda corydoras manage on smooth fine gravel, but sterbai corydoras need sand.

safe

Bare bottom (quarantine and grow-out only)Legitimate for quarantine and grow-out tanks. Easy to clean, but it holds far less biological filtration, and it is no long-term home for burrowing loaches or sand-sifters.

safe

With a caveat

Crushed coral or aragoniteDeliberately raises hardness and pH. Correct for rift-lake cichlids, wrong for soft-water species.

with a caveat

Aquasoil for planted tanksExcellent for plants, but most active soils soften the water and pull pH and KH down for months, and many leach ammonia for the first weeks. Right for soft-water species; wrong for hard-water animals (livebearers, rainbowfish, goldfish, and snails, whose shells erode in soft, acidic water) unless you test GH and KH and remineralise. Cycle around it; do not stock into it.

with a caveat

Very deep sand bedsAnaerobic pockets form below about 2 inches in still sand. Either keep it shallow or keep it stirred.

with a caveat

Painted or coated gravelUsually fine, but coatings chip. Test a handful in a jar for a week first.

with a caveat

Never

Sharp or angular gravel with bottom-dwellersWears down barbels and leads to infection in corydoras and loaches.

never

Anything containing metal, limestone of unknown origin, or shells in a soft-water tankSlowly alters chemistry in ways that are hard to diagnose later.

never

This is our general list for freshwater fish. We have not built one specific to Rainbowfish yet.

Top resources for more info

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