Fellow Creatures
Pet match quiz
Harlequin Rasbora

NasserHalaweh · CC BY-SA 4.0 · Wikimedia Commons (cropped)

Freshwater Fish · Rasboras

Harlequin Rasbora

Trigonostigma heteromorpha

Copper shoaler with a black triangular patchBeginner

Lifespan
5-8 years
Size
1.5-2 in
Social needs
Shoaler; keep 12+ together
Time
Low; daily feeding, weekly water change
Cost
Low
Diet
Omnivore; micro pellets, flakes, frozen foods

Overview

What is this animal?

  • The harlequin rasbora is a peaceful shoaling fish from the soft, acidic forest streams and peat swamps of Southeast Asia.
  • Its warm copper-pink body and black triangular patch make a dense shoal an attractive sight in a planted community tank.
  • Hardy, long-lived and captive-bred, it is a good beginner fish that adapts well once a tank is cycled.
  • It rewards patience with years of reliable colour and calm behaviour in the right soft-water setup.

Temperament

  • Peaceful, social and tightly shoaling when kept in good numbers, harlequins glide calmly through midwater and rarely trouble their tankmates.
  • A larger group settles faster, colours up and swims more openly.
Show 2 more
  • They are good community fish alongside other gentle soft-water species such as small tetras, corydoras, dwarf cichlids and shrimp.
  • Avoid large, boisterous or predatory tankmates that intimidate the shoal or keep these mild fish hiding among the plants.

A good fit for

  • Beginners wanting a peaceful, long-lived shoal
  • Planted soft-water community aquariums
  • Keepers seeking calm, reliable colour
  • Shrimp-friendly and biotope-style tanks

Common mistakes to avoid

  • Keeping too few, causing chronic stress
  • Hard, alkaline water dulling their colour
  • Adding them to an immature, uncycled tank
  • Housing with large or aggressive tankmates

Enclosure

Minimum 16 gallons, for a group of 12

Cannot be kept alone. The minimum is 12, and the space above is for that group.

  • Keep a shoal of twelve or more in a tank at least 24 in long holding 16 US gallons or more, filtered and heated to 73-81°F.
  • They look best against dark substrate in a planted, dimly lit tank with driftwood and light tannin staining that echoes their native swamps.
  • They prefer soft, slightly acidic water but adapt to moderately hard supplies if kept stable.
  • Provide gentle flow, open midwater swimming space and planted cover so the shoal feels secure enough to display full colour.

Check your enclosure

Supplies

Setup

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

Ongoing

  • Quality micro-pellet or fine flake
  • Frozen daphnia and cyclops
  • Frozen or live baby brine shrimp
  • Frozen bloodworm (to be chopped fine)
  • Small scissors or blade for chopping frozen food
Show 2 more
  • Fine net for removing leftovers
  • Small feeding spoon

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 a fine-grained food
  • Head-count and look everyone over

Every few days

  • Top up evaporationevery 3 days
Show 8 more

Weekly

  • Count the school
  • 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 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 review with a new-stock check, every 3 months

Tools

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

Care in depth

Diet

Omnivore; micro pellets, flakes, frozen foods
  • Harlequins are easy omnivores with small mouths.
  • A quality micro-pellet or flake forms a dependable staple, rotated with frozen or live daphnia, baby brine shrimp, cyclops and finely chopped bloodworm for variety and condition.
  • Feed small amounts once or twice daily, only what is consumed within a minute or two.
  • Varied feeding supports their colour and the general health that helps them reach their long lifespan.

Health

Signs of good health, and the common problems
  • Harlequins are robust and disease-resistant, with most problems traceable to chilling, poor water quality or stress rather than weak stock.
  • Ich and fin rot are the usual concerns and respond quickly to corrected conditions and prompt treatment.
  • Stable warmth, soft water and weekly partial changes prevent the majority of issues.
  • Quarantine new arrivals, acclimatise gently, and keep them in a settled, well-cycled tank, where they prove trouble-free over many years.

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 Rasboras yet.

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 2026-10-22.

YouTubers

Girl Talks FishCare how-tos by fish type, tank setup and beginner mistakes (the host works for Aquarium Co-Op, a retailer)Aquarium Co-OpSpecies care guides, filtration and fish health (run by a retailer)Prime Time AquaticsSpecies profiles, stocking and water chemistry explained plainly

Sites

Keep going

Rasboras, side by side

Same axes, same care guides: the honest way to choose between types.

See all Rasboras side by side →Compare with other animals

More Freshwater Fish guides

Chili Rasbora
Boraras brigittae
Intermediate

Chilli-red speck from Borneo's peat swamps

4-8 yearsRead guide →
Long-finned Betta
Betta splendens
Beginner

Flowing-finned labyrinth fish that lives alone

3-5 yearsRead guide →
Short-finned Betta
Betta splendens
Beginner

The short-finned plakat: an athletic betta that lives alone

3-5 yearsRead guide →
Guppy
Poecilia reticulata
Beginner

Colourful livebearer that breeds readily

2-3 yearsRead guide →
Endler's Livebearer
Poecilia wingei
Beginner

Tiny, colourful livebearer for small planted tanks

2-3 yearsRead guide →
Platy
Xiphophorus maculatus
Beginner

Hardy, colourful community livebearer

3-4 yearsRead guide →

Is your enclosure big enough?

These are the smallest sizes that give good conditions, not legal minimums. More room is always better.

Minimum 16 gallons, for a group of 12

Every figure, and where it comes from
Minimum volume16 gallons
Swimming length24 in minimum. A tall tank of the right volume is still the wrong shape.
CompanyCannot be kept alone. The minimum is 12, and the space above is for that group.
The recognised standards do not agree on this one
Practical Fishkeeping (the genus Trigonostigma): A small group in a 24-inch tank; around 16 gal for its smaller relative, the glowlight rasbora. The figure this site uses: a 24-inch tank, with at least the 16 gallons given for the smaller species. Checked 2026-10-23.
Seriously Fish (Trigonostigma heteromorpha): 24 x 12 in base, about 14 US gal. The same length: the smallest Seriously Fish says to consider, because this fish lives in numbers. Checked 2026-10-23.
Practical Fishkeeping: No less than 24 in long, with at least 12 fish. The same length, and the group size used here. Checked 2026-10-23.
Austrian legal minimum (2. Tierhaltungsverordnung, Anlage 5): 24 x 12 x 12 in (14 gal). A legal minimum, recorded for context. Checked 2026-10-23.

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.

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.