Harlequin Rasbora
Trigonostigma heteromorpha
Min Tank Size
60L
Adult Size
4.5 cm
Lifespan
5 years
School Size
6+
About
Native to the blackwater rivers and peat swamps of Southeast Asia, particularly the Malay Peninsula, Sumatra, and Thailand, harlequin rasboras have been a staple of the hobby for decades. And honestly, it's easy to see why.
That warm copper-orange body paired with the bold black triangular wedge running from mid-body to the tail gives them a look that holds up even next to flashier modern imports. They're not loud fish, but a school of twelve moving through a well-planted tank catches the eye every time.
Care is genuinely straightforward. They're hardy enough for beginners but reward more experienced keepers who dial in soft, slightly acidic water. Somewhere in the pH 6.0 to 7.0 range with low general hardness gets you the richest coloration and the best long-term health. They'll tolerate harder tap water, but colors can look a bit muted. Temperature-wise, 24 to 27 Celsius hits a sweet spot. Filtration should be reliable but not blasting current, since these fish come from slow-moving jungle streams. Feeding is easy. Quality flake or micro-pellet covers the base, and they go absolutely nuts for small frozen or live foods like daphnia, baby brine shrimp, or bloodworm. A varied diet keeps them in prime condition.
Temperamentally, there's almost nothing to warn about. They coexist peacefully with practically everything of a similar size, and their schooling behavior is some of the most natural-looking you'll find in the hobby. Groups of eight or more school tightly and move with real cohesion, especially when there's a slight current to push against.
Search tank builds featuring harlequins and you'll find them popping up in everything from minimalist Walstad setups to lush Dutch-style planted tanks.
Water Parameters
Temperature
°CpH
GH
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dKHSwimming Level
Flow Preference
Keeping multiple Harlequin Rasbora together
Harlequin Rasbora are shoaling fish and need company of their own kind. Keep a group of at least 6. Smaller groups leave them stressed, washed-out in color, and prone to hiding.
Compatibility
These fish work with a huge range of tankmates. Corydoras catfish, small loaches like kuhlis, otoclinus, dwarf gouramis, and virtually any small peaceful tetra make excellent companions. They're commonly paired with ember tetras or chili rasboras for a natural Southeast Asian look. Avoid anything large enough to eat them, or fish with strong fin-nipping tendencies like tiger barbs or serpae tetras. As for invertebrates, adult dwarf shrimp like neocaridinas are generally fine, but expect occasional losses of very small shrimp fry since harlequins won't pass up tiny moving things at feeding time. Snails are completely safe.
Commonly kept with
Species this one is most often paired withCommonly tried but avoid
Often paired, but shouldn't beCare Notes
The most common mistake is keeping too few. A group of three or four looks stressed, hides constantly, and never schools properly. Six is the minimum that produces natural behavior, and eight or more is where they really shine. Second issue is water hardness. Harlequins from a soft-water source placed straight into hard tap water often survive but never look their best. If your tap is very hard, consider blending with RO or using a blackwater conditioner. Overfeeding is also worth watching since they're enthusiastic eaters but their bioload adds up in smaller tanks.
Behavior & Aggression
Harlequin rasboras are about as non-aggressive as a fish can get. There's no meaningful territory-holding, no fin nipping, and no significant conspecific conflict even in smaller tanks. Occasionally two males will display briefly toward each other during spawning condition, but it amounts to a bit of parallel swimming and nothing more. Keeping a larger group actually reduces any mild competition, since attention gets spread around and no individual gets singled out. There are no known triggers for aggression in a normal community setting.
Things to Know
- Groups smaller than 6 show stress and washed-out color.
- May snack on dwarf shrimp fry, though adults are usually left alone.
- Prefers soft, slightly acidic water for best color and breeding condition.
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