Should I buy it?
Harlequin Rasbora
Trigonostigma heteromorpha
Also known as: harlequin, trigonostigma heteromorpha
Easier than neons in moderately hard water. The standard pick for a first community tank where neons would struggle.
Harlequin Rasbora tank mates · 60L tank size hub
peaceful · beginner care
Sources & confidencehigh · verified · beginner: excellent
Checked against several solid care sources that mostly agree. Numbers here are a careful starting point. Your tap water, tank size, and habits decide the rest. How we evaluate.
Best for: Almost any peaceful community 60L+. They get along with practically everything calm.
Avoid if: Your tank is under 50L, or you have predators that will eat 4cm fish.
Common mistakeBuying four harlequins as a starter school is how the species earns a dull reputation. The group only develops full colour and tight schooling with roughly eight or more in settled water. Token numbers hide under plants and never look like the shop display. Budget for a real school up front, or wait.
Top things that go wrong
- Group welfare — not a solo display fish. Plan at least **6** together for normal behaviour; smaller groups often mean stress, colour loss, or nipping depending on species.
- Shrimp & snails. Cherry shrimp and other dwarfs can coexist with peaceful small fish; baby shrimp are food for almost anything that finds them.
What most shops don't tell you
- 1.Harlequins are sold as the universal calm community fish that works in any small tank. They are peaceful, but they still need a proper school and freedom from predators that eat 4 cm fish. A handful in a bare 40-litre cube is not the ownership plan on the label. Recreate the school and the planting, or the calm disappears.
- 2.Too few in a 40L. A stressed harlequin packs under wood or loses colour within days.
- 3.Housing with coldwater goldfish. A tropical rasbora reads as a stressed outline at the back.
- 4.An easier school than neons in moderately hard water. Six or more, or they hide in the back. Tap water in most cities works.
About this species
Harlequin rasboras are small soft-water schoolers from Borneo and Malaysia. The black wedge behind the dorsal anchors the school visually.
- Cherry Barb60L min · same group, comparable tank size
- Dwarf pencilfish60L min · same group, comparable tank size
- Lambchop / Espei rasbora60L min · same group, comparable tank size
- Zebra Danio60L min · same group, comparable tank size
- Golden / Beckford's pencilfish80L min · same group, similar adult size
- Pearl Danio80L min · same group, similar adult size
- White Cloud Mountain Minnow40L min · same group, similar adult size
- Celestial Pearl Danio30L min · same group, similar adult size
- Amano Shrimpalso beginner peaceful, similar tank size
- Assassin Snailalso beginner peaceful, similar tank size
- Beckford Pencilfishalso beginner peaceful, similar tank size
- Black Neon Tetraalso beginner peaceful, similar tank size
- Black phantom tetraalso beginner peaceful, similar tank size
- Bloodfin tetraalso beginner peaceful, similar tank size
- Apistogramma Borellii tank matesnames this fish as a conditional pairing — read the pair page conditions
- Assassin Snail tank mateslists this fish as a safe and recommended mate
- Bamboo Shrimp tank mateslists this fish as a safe and recommended mate
- Black Neon Tetra tank mateslists this fish as a safe and recommended mate
- Black phantom tetra tank mateslists this fish among its recommended pairings
- Black ruby barb tank mateslists this fish among its recommended pairings
- Bloodfin tetra tank mateslists this fish among its recommended pairings
- Blue Dream Shrimp tank mateslists this fish as a safe and recommended mate
Plan grid
Key limits are above. Here: pH band, swim level, bioload, and activity.
Swim zones
Care snapshot
Quick compare before you buy. Not a score of how “good” the fish is.
- Beginner ease85
- Peacefulness90
- Community fit86
- Small-tank fit100
- Hardiness79
- Energy54
Numbers are deterministic planning indices from Fishori fields — not a scientific score of your individual fish.
Example setups
Starting points only. Run the pair checker for every fish you add.
Rough 90L+ layout: one calm centrepiece, 8–12 small tetras/rasboras, 6–8 corydoras-type bottom fish — verify every name in the pair checker before buying.
Safer directions include: Cherry Barb, Corydoras Catfish, Dwarf Gourami.
Prioritise 6+ of Harlequin Rasbora in 60L+ with filtration sized for messy feeding — add only mates that already pass pair checks with this species.
Compatibility
Tank mates for Harlequin Rasbora
Usually safer (engine GOOD): Cherry Barb, Corydoras Catfish, Dwarf Gourami, Guppy, Neon Tetra. Conditional (CAUTION — read the pair page before buying): Tiger Barb. Avoid (RISKY): Jack Dempsey, Oscar. Every name links to a live pair check — the lists are a shortlist, not a stocking plan.
Usually safer
Open a pair page with this fish as the starting point.
Needs planning
Read the blocking rule on each pair page before experimenting.
Usually avoid
Do-not-stock combinations on conservative hobby rules.
Compare with
Run a real pair check: Harlequin Rasbora + Cherry Barb
- Try Cherry Barb — open the pair check.
- Try Corydoras Catfish — open the pair check.
- Try Dwarf Gourami — open the pair check.
- Try Guppy — open the pair check.
Behaviour and temperament
Harlequin rasboras are calm midwater schoolers that actually hold a group shape in a planted tank when numbers are honest. Eight or more usually colour up and move as a band; four fish look like nervous individuals. They do not start fights with peaceful neighbours of similar size. Predators and fin-nippers are the usual failure mode, not harlequin aggression.
What sets them off
- Sudden crowding
- Poor water quality
Fin nipping: Not a habitual fin-nipper; stressed or crowded individuals can still test fins.
Predation: Not a predator toward similarly sized community fish; fry and dwarf shrimp are still fair game for most species.
Territory: Not strongly territorial, but still needs room so its working space does not overlap the next fish.
Planted tanks: excellent — easy plant ideas
In the glass: typical and warning signs
- Midwater school that settles into a calm triangle-pattern shoal in planted tanks.
- Eight or more shows the real behaviour; four looks like lost fish.
- Hiding in the back after a parameter swing or with boisterous tank mates.
- Colour fade in hard, bright, bare water.
- Peaceful; in-school sorting only.
- School never comes out → calm the traffic above them and add cover before rehoming.
- One fish left behind and dark → quarantine.
Fish behaviour can vary between individuals and tank setups. Always observe new fish closely after introduction.
Water, food, and grouping
Hardness
soft
Diet
omnivore
Harlequins take small flake and micro-pellets without drama, but they still lose out to frantic surface feeders. Feed a pinch they can reach in midwater, and add tiny frozen foods a few times a week so colour holds. They are not algae crew. If the school stays pale while eating, check numbers and cover before changing brands again.
Shrimp & snails
Cherry shrimp and other dwarfs can coexist with peaceful small fish; baby shrimp are food for almost anything that finds them.
Schooling species — buy 6+ together. Smaller groups stress and lose colour.
- Hold 22 to 27 °C steadily on a real thermometer, not the dial on the heater.
- Aim for pH 6 to 7.5 and a hardness you can re-test in two weeks. A one-time strip in the shop car park is not a water test.
- Schooling species. Buy 6 or more from the same tank on the same day before adding any centrepiece fish.
- Tank volume meets or exceeds 60L published minimum for adults.
- You can stock at least 6 individuals (group welfare).
- Heater can hold 22–27°C without cooking cooler-water tank mates.
Explore and stocking hubs
Tank-mates hub, matching guides, category peers, and litre stocking lists around this species’ minimum tank.
Plan with tools
Check mates as pairs first, then add this fish in the tank builder for the full stocking.
Filtration & heating
A 60L minimum tank needs a filter rated for at least 240L/hr turnover and a heater to hold 22–27°C reliably.
Plant suggestions
Harlequin Rasbora does well in planted tanks. Plants compatible with 22–27°C and pH 6–7.5:
Research: verified · Confidence: high · 3 linked source(s). We do not invent citations.
How Fishori evaluates compatibility (same logic as pair and tank tools).
- Seriously Fish. Trigonostigma heteromorpha
Supports: aquarium size, water chemistry, temperament and compatibility (primary care reference).
- FishBase. Trigonostigma heteromorpha
Supports: taxonomy and wild maximum size — translate to tank stocking, not a target number.
- Wikipedia. Trigonostigma heteromorpha
Supports: general species background and common-name orientation only.
Evidence notes
- Care numbers are cross-checked across the linked sources and biased toward the conservative end; compatibility reflects typical hobby experience, not a lab prediction. Method and full source policy: /methodology.
Featured compatibility checks
Curated pair pages for Harlequin Rasbora. Each opens a full verdict — not just a list label.
- Harlequin Rasbora + Corydoras Catfish
- Harlequin Rasbora + Neon Tetra
- Harlequin Rasbora + Angelfish
- Harlequin Rasbora + Betta
- Harlequin Rasbora + Cardinal Tetra
- Harlequin Rasbora + Cherry Barb
- Harlequin Rasbora + Dwarf Gourami
- Harlequin Rasbora + Ember Tetra
- Harlequin Rasbora + German Blue Ram
- Harlequin Rasbora + Oscar
- Harlequin Rasbora + Tiger Barb
- Harlequin Rasbora + Bamboo Shrimp
Common questions about Harlequin Rasbora
- How big do Harlequin Rasbora get?
- Adults reach 4 to 5 cm. The black wedge holds the school together visually, even at six fish.
- What size tank does Harlequin Rasbora need?
- 60L is realistic for a school of eight or more. Smaller tanks limit lateral schooling.
- Are Harlequin Rasbora aggressive?
- Harlequin rasboras are calm midwater schoolers that actually hold a group shape in a planted tank when numbers are honest. Eight or more usually colour up and move as a band; four fish look like nervous individuals. They do not start fights with peaceful neighbours of similar size. Predators and fin-nippers are the usual failure mode, not harlequin aggression.
- Do Harlequin Rasbora need to be kept in a group?
- Schooling species — buy 6+ together. Smaller groups stress and lose colour.
- What is the most common mistake with Harlequin Rasbora?
- Buying four harlequins as a starter school is how the species earns a dull reputation. The group only develops full colour and tight schooling with roughly eight or more in settled water. Token numbers hide under plants and never look like the shop display. Budget for a real school up front, or wait.
- What fish can live with Harlequin Rasbora?
- The Harlequin Rasbora profile lists Cherry Barb, Corydoras Catfish, Dwarf Gourami, Guppy among its safer pairings (engine-checked GOOD). Tiger Barb is conditional (CAUTION): workable only with the specific conditions on its pair page. Avoid Jack Dempsey, Oscar (RISKY). Run the pair checker for your exact water and tank size before stocking.