How many harlequin rasbora should I keep?

Minimum 6; for community-tank stability and proper behavior, the high end of the range is more reliable. Solo or paired individuals of schooling species typically hide and stop displaying normal behavior.

What tank size do I need?

Realistic minimum is 20 gal long. Smaller tanks technically house the species but compress the social and territorial dynamics in ways that show up as stress over time.

What water parameters does this species prefer?

Temperature 73–82°F (23–28°C), pH 6.0–7.5, GH 2–12 dGH. Tank-bred commercial stock typically tolerates a wider range than wild-type fish, but parameters in the middle of the listed range produce the most consistent behavior.

Harlequin Rasbora Care — The classic peaceful community schooler with the i

Scientific name

Trigonostigma heteromorpha

Origin

Malaysia, Singapore, Thailand, Sumatra

Adult size

1.5–2 in (3.8–5 cm)

Lifespan

5–8 years

Min group / structure

6 (8–10 better)

Min tank size

20 gal long

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Overview

Mid-water schoolers, calm and hardy, one of the most reliable starter species for a planted community tank. Loose-shoaling but distinctly more cohesive in groups of 8+.

Behavior and group dynamics

Harlequins shoal mid-water and stay confidently in the open when housed in groups of 6+. Coloration is most saturated when the tank has dark substrate, dim lighting, and live plants — they evolved in shaded blackwater habitats. Behavior is non-aggressive; the species is genuinely community-safe with anything its own size or smaller, including shrimp.

Tank requirements

A 20-gallon long is the realistic floor for a school of 6. Larger groups (10+) in a 29-gallon or 40-breeder display the iconic synchronized swimming pattern most reliably. Gentle filtration; harlequins are not strong swimmers and prefer moderate current.

Temperature: 73–82°F (23–28°C).
pH: 6.0–7.5.
GH: 2–12 dGH.
Ammonia / nitrite: 0 ppm sustained.
Nitrate:< 20 ppm.

Tank mates

Compatible with:

Other small peaceful tetras (neon, ember, cardinal)
Corydoras catfish
Otocinclus
Cherry barbs
Mystery snails / nerite snails
Adult cherry shrimp (fry will be eaten)

Avoid:

Aggressive cichlids
Adult angelfish (size mismatch)
Tiger barbs (fin-nipping)

Frequently asked questions

How many harlequin rasbora should I keep?: Minimum 6; for community-tank stability and proper behavior, the high end of the range is more reliable. Solo or paired individuals of schooling species typically hide and stop displaying normal behavior.
What tank size do I need?: Realistic minimum is 20 gal long. Smaller tanks technically house the species but compress the social and territorial dynamics in ways that show up as stress over time.
What water parameters does this species prefer?: Temperature 73–82°F (23–28°C), pH 6.0–7.5, GH 2–12 dGH. Tank-bred commercial stock typically tolerates a wider range than wild-type fish, but parameters in the middle of the listed range produce the most consistent behavior.

Stocking density calculator — verify community compatibility before adding harlequin rasbora.
Tank volume calculator — confirm working volume vs nominal tank size.
Cycling a new aquarium — non-negotiable before adding any fish.
Water parameters explained — context for the temp / pH / GH ranges above.

Not veterinary advice — for sick fish or tank emergencies, consult an aquatic veterinarian or a qualified local aquarium professional.

Sources: FishBase species page. Mainstream hobby references (Aquarium Co-Op, Tetra, Hagen care sheets) used for hobbyist-context framing only — not cited as anchors for numerical claims. Where sources diverged, this guide takes the conservative beginner-safe position.

Harlequin Rasbora care