At a glance the quick version
Looks how to spot one
Small scaled cyprinid with a deep, diamond-shaped body
A compact, lozenge-shaped fish with a warm copper to pinkish-orange body. Its signature mark is a bold black triangle on the rear half of the body that narrows towards the tail. The fins carry a reddish tint, and the forked tail has red outer edges with clear centres. Males tend to show richer colour and a sharper, slightly larger black patch, while females are rounder.321
Personality what they are like
An exceptionally gentle and sociable fish that rarely bothers anything else in the tank. It is at its most confident and colourful when it can swim with a decent-sized group of its own kind.23
Daily activity how busy a day looks
An active swimmer that spends its time in the open middle area of the tank. It enjoys calm to gently moving water with plenty of plants around the edges and free space to shoal.21
Grooming and care the hands-on bits
Keep the water soft, warm and slightly acidic to neutral, with regular partial water changes. Leaf litter, driftwood and subdued lighting mimic its peat-swamp home and bring out its colour.23
Home where they are happiest
Aquarium with a footprint of at least 60 x 30 cm (about 54 litres). Temperature around 21-28 C (FishBase gives 22-25 C), pH about 5.0-7.5 (FishBase 5.0-7.0), hardness roughly 1-12 dGH. A well-planted tank with some broad-leaved plants and open swimming water suits it best.21
Food basics what goes in the bowl
In nature it hunts small insects, worms, crustaceans and other tiny animals. In the aquarium it readily takes quality flake and small granules, and benefits from regular live or frozen treats such as daphnia, bloodworm, brine shrimp and mosquito larvae.213
Company who they like around
A schooling species that should live in a group; Seriously Fish suggests at least 8-10, Wikipedia at least 6 and FishBase 5 or more. Good tankmates are other small, peaceful fish of similar size.231
The story where it all began
The harlequin rasbora comes from slow forest streams and peat swamps of the Malay Peninsula and nearby islands, where the water is stained brown by humic acids and very low in minerals, much like South American blackwater. It was described by the German zoologist Georg Duncker in 1904 as Rasbora heteromorpha; in 1999 Kottelat and Witte moved it into the genus Trigonostigma, whose name points to its triangular marking. The fish reached European aquarists in the early 1900s and became a favourite almost at once. Its standing in the hobby was cemented when its image, stamped in gold, appeared on the cover of William T. Innes's classic book Exotic Aquarium Fishes in 1935, staying there through all 19 editions. It remains one of the most familiar and long-lasting species in fishkeeping.23
Fun facts for the dinner table
- 1
Unlike most small cyprinids, it does not scatter its eggs: the pair turn upside down beneath a broad leaf and stick the eggs to its underside.2
- 2
A single spawning can produce roughly 80 to 300 eggs, which hatch in about 18 hours at breeding temperatures.3
- 3
Its image was stamped in 14k gold on the cover of the first edition of William T. Innes's 1935 aquarium classic, and stayed for all 19 editions.3
- 4
The species name heteromorpha comes from Greek for 'differently shaped', a nod to its deep body compared with other rasboras.3
- 5
In the wild it can be found in large groups of more than 100 fish.1
- 6
Recent genetic studies hint that what we call the harlequin rasbora may actually include more than one closely related species.2
- 7
Its close relatives in the same genus, such as the lambchop rasbora (T. espei) and T. hengeli, look similar but have a differently shaped dark marking.3
Recognised by the official bits
- IUCN Red List
- Assessed as Least Concern (January 2019)1
- FishBase
- Listed as highly commercial in both fisheries and the aquarium trade1
The care rhythm Furry Diary remembers it for you
A typical home-care rhythm for a Harlequin Rasbora. It is our own starting plan, not a vet's advice, and you can change every line in the app.
- FeedingSmall amounts once or twice a dayevery day
- Temperature checkevery day
- Water changeAbout 25% of the waterevery week
- Water testevery week
- Glass cleanevery week
- Filter rinseRinse in old tank water, never tap waterevery month
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Where this comes from
Every breed page shows where each fact comes from. If we got something wrong, sorry, we are pet people, not a science magazine. Tell us at support@furrydiary.app and we will fix it.
- FishBase: Trigonostigma heteromorpha fishbase.seused for max length, distribution, habitat, water parameters, diet, IUCN status, group size
- Seriously Fish: Trigonostigma heteromorpha seriouslyfish.comused for size, tank size, water parameters, diet, behaviour, breeding, habitat, taxonomy, colour strains
- Wikipedia: Harlequin rasbora en.wikipedia.orgused for description, size, lifespan, etymology, breeding, aquarium history
Checked 2026-09-30. Text is written in our own words from these sources.