Harlequin Rasbora
Trigonostigma heteromorpha
Harlequin rasboras are those little coppery-orange fish with the bold black "wedge" on their sides that somehow look even better once they're cruising in a group. Give them a nice school and some plants to weave through and they'll do this tight, synchronized swimming thing that's honestly kind of hypnotic.
The Harlequin Rasbora features a distinctive triangular black patch above its caudal fin, complemented by a vibrant orange body and shimmering scales.
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Quick Facts
Size
2 inches
Temperament
Peaceful
Difficulty
Beginner
Min Tank Size
10 gallons
Lifespan
5-8 years
Origin
Southeast Asia
Diet
Omnivore - small pellets/flake, plus frozen/live foods like daphnia, brine shrimp, and bloodworms
Water Parameters
22-28°C
6-7.5
2-12 dGH
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Calculate heater sizeCare Notes
- Get at least 6-10 harlequins (more is better); a small group looks stressed and hides, but a proper shoal stays out and schools nicely.
- A 15-20 gallon tank works great for a group, with plants (real or fake) and some open swimming room- they love cruising the midwater.
- Keep them in the mid-70s °F (around 24-26°C) and aim for slightly acidic to neutral water (about pH 6.0-7.2); they're way happier with stable temps than chasing "perfect" numbers.
- Feed small foods they can finish quickly: quality micro pellets or flakes as the staple, and toss in baby brine shrimp/daphnia/frozen cyclops a few times a week for color and energy.
- They're super peaceful-great with other chill fish like tetras, corys, small gouramis, and shrimp; skip fin-nippers (some barbs) and anything big enough to see them as snacks.
- Use a lid or keep the water level a bit down; they can jump when spooked, especially right after you turn lights on or do maintenance.
- If one starts clamping fins, fading, or hanging by the filter, check ammonia/nitrite first and do a water change-these guys don't tolerate "new tank" problems well.
- Breeding is doable but a bit picky: they scatter eggs under broad leaves (Crypts are classic), and the adults will eat the eggs, so you'll need a separate breeding tank or pull the parents after spawning.
Compatibility
Good Tankmates
- Other small, chill schooling fish (think ember tetras, rummy-nose, glowlight tetras) - they all vibe the same way and nobody tries to boss the tank around.
- Corydoras catfish - perfect roommates since cories do their own thing on the bottom and harlequins cruise the middle. Zero drama.
- Small, peaceful loaches like kuhli loaches - they're shy and mostly nocturnal, so they won't compete or stress the rasboras out.
- Dwarf gouramis / honey gouramis - calm centerpiece fish that usually ignore rasboras, and the rasboras help them feel less skittish.
- Peaceful dwarf shrimp and snails (amano shrimp, nerites) - rasboras are generally too small-mouthed to be a big shrimp problem, especially with some plants and cover.
- Peaceful livebearers like platies (and sometimes guppies) - works if the tank isn't too hard/alkaline and nobody's fin-nippy; they mostly stay out of each other's way.
Avoid
- Fin-nippers like tiger barbs (and even some "spicy" black skirts/serpaes) - they'll hassle the rasboras and keep the whole school on edge.
- Bigger semi-aggressive stuff (cichlids like convicts, larger barbs) - harlequins are peaceful and will get pushed around or straight-up eaten when they're small.
- Anything predatory that sees a 1.5-2 inch fish as a snack (most medium/large gouramis, larger catfish, puffers) - if it can fit a rasbora in its mouth, it's a bad plan.
1) Where they come from (quick backstory)
Harlequin rasboras come from slow, plant-choked waters in Southeast Asia—think Malaysia, Thailand, Singapore. A lot of the places they live are stained tea-brown from leaves and wood, with gentle flow and loads of cover. That vibe (dim light, plants, calm water) is basically the cheat code for keeping them looking their best.
2) Setting up their tank
If you want them confident and out in the open, give them a decent group and a tank that isn’t bare. They’re small fish, but they act way braver when they have buddies and places to weave in and out of.
- Tank size: 10 gallons can work for a small group, but 20 long is where they start to look really natural and school nicely
- Group size: 8–12+ is the sweet spot; 6 is the bare minimum in my experience
- Temp: mid-70s to around 80°F (24–27°C)
- pH/hardness: they’re pretty flexible, but slightly acidic to neutral and on the softer side keeps them happiest
- Filter/flow: gentle to moderate; avoid blasting them with a powerhead-style current
- Layout: plants (especially mid/background), some open swimming room, and darker substrate if you want better color
- Lighting: not super bright; floating plants or tannins calm them down fast
If your rasboras hide a lot, it’s usually one of three things: group too small, tank too bright/bare, or tankmates too pushy. Fix those and they change overnight.
They don’t need blackwater conditions, but they do appreciate a “soft” look—driftwood, leaf litter, floating plants. I’ve kept them in clear water too, but they colored up and schooled tighter in a slightly dim, planted setup.
3) What to feed them
Harlequins are easy eaters. They’ve got small mouths, so you’ll get better results with smaller foods rather than big flakes that they have to spit and chase.
- Staples: a quality micro pellet and/or crushed flake
- Frozen: baby brine shrimp, cyclops, daphnia, finely chopped bloodworms (as a treat)
- Live (if you can): baby brine shrimp is basically magic for color and conditioning
- Feeding rhythm: small portions once or twice a day; they’re enthusiastic and will beg like they’re starving
Overfeeding shows up fast with these guys—bloated bellies and dirtier water. If they’re leaving food drifting to the bottom, you’re giving too much.
4) Behavior and tankmates
They’re classic peaceful schooling fish. In a good-sized group they’ll do that little “follow the leader” cruising pattern, and the males will spar without causing damage—more like showing off than fighting.
- Great tankmates: other small peaceful schooling fish, dwarf corys, otos, small loaches, peaceful gouramis, shrimp (adult shrimp usually fine)
- Also good: most community-friendly tetras if your water parameters overlap
- Skip: fin nippers (tiger barbs), big/boisterous fish, or anything that sees small fish as snacks
- With bettas: sometimes works in larger, planted tanks, but watch for chasing or stressed rasboras hiding
Harlequins aren’t notorious jumpers, but I’ve still had the occasional surprise. If your tank has gaps, a lid or some floating plants is cheap insurance.
5) Breeding tips (if you feel like trying)
Breeding harlequins is doable, but it’s not as “easy mode” as livebearers. The fun part is their spawning behavior: they like to lay eggs on the underside of broad leaves (Crypts are the classic choice).
- Use a separate breeding tank if you actually want fry—adults will snack on eggs
- Softer, slightly acidic water helps a lot; warm temp and gentle filtration
- Add broad-leaf plants (Cryptocoryne, Anubias, or even a clean, broad artificial leaf)
- Condition with live/frozen foods for a week or two
- Dim lighting and some cover seem to encourage spawning
If you see a pair hanging around a leaf and doing little “practice” turns, you’re close. After spawning, pull the adults or move the leaf with eggs to a rearing tank.
Fry are tiny at first. You’ll need appropriately small foods (infusoria/rotifers, then baby brine shrimp once they’re big enough). If that sounds like a hassle, enjoy them as display fish—nothing wrong with that.
6) Common problems to watch for
Most issues with harlequins aren’t exotic—they’re usually the same community-tank headaches: stress, water quality swings, and new-fish sickness.
- Faded color/hiding: tank too bright, not enough cover, group too small, or pushy tankmates
- Ich and other parasites: often shows up after adding new fish; quarantine helps a ton
- Mouth too small for food: big flakes get spit out—switch to micro pellets or crush the flake
- Wasting away/skinny fish: can be internal parasites or competition at feeding time; watch that everyone actually eats
- Sudden losses after a water change: they don’t love big parameter swings—match temp and don’t do massive changes if your tap is very different from tank water
They’re tougher than people give them credit for, but they hate “new tank chaos.” Let the tank mature, keep nitrates reasonable, and avoid constant fiddling. Stable beats fancy.
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