Freshwater Fish
Boeseman's Rainbowfish
Melanotaenia boesemani
Half blue, half orange, all motionIntermediate
- Lifespan
- 5-8 years
- Size
- Males 10-11cm (4-4.5in); females to 8cm
- Social needs
- Shoaler; keep 6-8+ with both sexes
- 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 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.
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.
A good fit for
- Keepers with a 4ft (120cm) 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 4ft 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 200 litres (53 US gallons) — for a group of 6
Cannot be kept alone. The minimum is 6, and the space above is for that group.
- 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.
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)
- Small scissors for chopping frozen food
- Fine net for removing leftovers
Safe substrate: Inert aquarium gravel · Fine, smooth sand · Aquasoil for planted tanks
Care routine
Daily
- Check temperature and equipment
- Feed
- Head-count and look everyone over
Weekly
- Top up evaporationevery 3 days
- Check for a thinning fish
- Clean glass and clear algae
- Test the water
- Water change and gravel vacuum
Monthly
- Trim and thin the plantsevery 2 weeks
- Check hardness as well as nitrate
- Check supplies and reagent dates
- Rinse the filter media
Less often
- Replace chemical filter media (Carbon/Purigen)every 2 months
Vet: Quarterly condition and water-hardness review — every 3 months
Top resources for more info
Independent creators and organisations we would send a friend to. We have no affiliation with any of them, and nothing here is sponsored. Links checked 2026-09-19.
YouTubers
Girl Talks FishCare how-tos by fish type, tank setup and beginner mistakesAquarium Co-OpSpecies care guides, filtration and fish health (run by a retailer)Prime Time AquaticsSpecies profiles, stocking and water chemistry explained plainly
Sites
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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.
Is your enclosure big enough?
Minimum volume200 litres (53 US gallons)
Swimming length120 cm minimum. A tall tank of the right volume is still the wrong shape.
CompanyCannot be kept alone. The minimum is 6, and the space above is for that group.
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.
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.
NowFish gasping at the surface, or crowding the filter outflowIncrease surface agitation immediately, then test the water. Either the oxygen has crashed or the gills are being chemically burned. Both are hours-scale problems.
NowSudden 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 point at the water or the equipment, not at a pathogen.
NowToday
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.
TodayWhite 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.
TodaySoon
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 daysNever 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.
Bedding and substrate
Substrate in an aquarium is a filter as much as a floor. Most of the biology lives in and on it.
Safe
Inert aquarium gravelRinse thoroughly before use. Grain size should be too large to swallow and too small to trap food.
safeFine, smooth sandCorrect for corydoras, kuhli loaches and anything that sifts. Their barbels wear away on sharp gravel.
safeAquasoil for planted tanksExcellent for plants, but it leaches ammonia for the first weeks. Cycle around it; do not stock into it.
safeBare bottomLegitimate for quarantine and grow-out tanks. Easy to clean, but it holds far less biological filtration.
safeWith a caveat
Crushed coral or aragoniteDeliberately raises hardness and pH. Correct for rift-lake cichlids, wrong for soft-water species.
with a caveatVery deep sand bedsAnaerobic pockets form below about 5 cm in still sand. Either keep it shallow or keep it stirred.
with a caveatPainted or coated gravelUsually fine, but coatings chip. Test a handful in a jar for a week first.
with a caveatNever
Sharp or angular gravel with bottom-dwellersWears down barbels and leads to infection in corydoras and loaches.
neverAnything containing metal, limestone of unknown origin, or shells in a soft-water tankSlowly alters chemistry in ways that are hard to diagnose later.
neverThis is our general list for freshwater fish. We have not built one specific to Rainbowfish yet.
My kit says 0.25 ppm. Is that dangerous?
A liquid kit measures total ammonia, but only the un-ionised fraction is really toxic, and that fraction is governed by pH and temperature — each whole pH unit multiplies it roughly tenfold. So the same reading can be nearly harmless in one tank and lethal in another; this works out which one you are in.
Compare with other animals
The same axes for every animal, side by side — the way the guides are written makes that a fair comparison rather than two articles.
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