Trigonostigma truncata (Truncate Harlequin Rasbora)

02 June 2026

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The Truncate Harlequin Rasbora, often referred to as the Blaustrich Keilfleckbarbe, is a captivating and peaceful schooling fish native to Southeast Asia. Known for its distinct black wedge-shaped marking and vibrant orange hues, this active cyprinid is highly popular among aquarists for its hardiness and striking appearance in planted aquariums.

Trigonostigma truncata (Truncate Harlequin Rasbora)

Overview

The Truncate Harlequin Rasbora (Trigonostigma truncata), frequently associated with the German common name Blaustrich Keilfleckbarbe, is a stunning freshwater fish that brings vibrant color and lively movement to any aquarium. Only recently distinguished from the classic Harlequin Rasbora (T. heteromorpha), it shares many of the same desirable traits, including a peaceful demeanor and a striking appearance. It is an excellent choice for both beginner and experienced aquarists looking to populate a peaceful community or biotope setup.

Image of blaustrich keilfleckbarbe rasbora truncata

Distribution and Habitat

This species is native to the tropical regions of Southeast Asia, primarily found along the Malay Peninsula and southern Thailand. They naturally inhabit sluggishly moving blackwater streams, peat swamps, and forest pools. These waters are typically heavily shaded by the forest canopy and stained brown with tannins from decaying leaves and submerged wood, resulting in soft, acidic water conditions.

Description of Truncate Harlequin Rasbora

Body shape:

They possess a somewhat deep, laterally compressed body that is roughly diamond or hatchet-shaped, making them look slightly bulkier than other slender rasbora species.

Color:

The base coloration is a translucent silver or clear tone, overlaid with a brilliant metallic orange or gold wash. Their most distinguishing feature is the dark black, wedge or lambchop-shaped patch that starts near the midsection and tapers toward the caudal peduncle.

Barbels:

Like many fish in their genus, they do not possess barbels.

Snout:

The snout is relatively short and slightly pointed, designed for picking small food items from the water column.

Eyes:

They have proportionally large eyes with a slightly reflective golden or reddish rim, aiding their vision in dimly lit blackwater environments.

Fins:

The dorsal and anal fins are mostly transparent but feature striking hints of yellow, gold, or bright orange, especially along the leading edges.

The Tail:

The caudal fin (tail) is deeply forked and generally transparent, often tipped with matching reddish-orange hues that complement the body's overall coloration.

Temper

This is a remarkably peaceful and somewhat timid species. They do not nip fins or bother other tank mates, making them model citizens for a community aquarium. However, their peaceful nature means they can easily be outcompeted or stressed by large, boisterous, or aggressive fish.

Sociality

They are a highly social, obligate schooling fish. To feel secure and display their best colors and natural behaviors, they must be kept in groups of at least 8 to 10 individuals. Larger schools will result in a spectacular display and significantly reduced stress levels.

Activity

Truncate Harlequin Rasboras are active mid-water swimmers. They spend the majority of their time darting gracefully through the middle and upper levels of the aquarium, especially when kept in a proper school.

Keeping and Caring for Truncate Harlequin Rasbora

Tank size:

A minimum tank size of 60 liters (about 15 gallons) is recommended to provide adequate swimming space for a proper school.

Water Parameters:

Temperature

They thrive in tropical temperatures ranging from 22°C to 28°C (72°F to 82°F).

pH

They prefer slightly acidic to neutral water, with an ideal pH range between 5.0 and 7.0.

Hardness

Soft water is best, ideally kept between 2 and 10 dGH.

Ammonia, Nitrite, Nitrate

Ammonia and Nitrite must always be at 0 ppm. Nitrates should be kept as low as possible, ideally below 20 ppm, through regular water changes.

Lighting

Dim to moderate lighting is preferred to replicate their shaded natural habitat. Floating plants can be used to diffuse bright aquarium lights.

Water flow

They prefer a gentle to moderate water flow. Harsh currents should be avoided as they originate from sluggish waters.

Decorations and Plants

A heavily planted tank with dark substrate brings out their best colors. Driftwood, botanical leaf litter (like Indian Almond leaves), and broad-leaved plants like Cryptocoryne or Anubias provide excellent hiding spots and mimic their natural blackwater environment.

Diet

They are omnivorous micro-predators in the wild, feeding on small insects, worms, and zooplankton. In the aquarium, they easily accept high-quality crushed flakes, micro-pellets, and freeze-dried foods. Supplement their diet with live or frozen foods such as daphnia, baby brine shrimp, and bloodworms for optimal health and coloration.

Some Feeding Tips

Because they have small mouths, ensure all food is appropriately sized. It is best to feed them small amounts twice a day, only providing what they can consume within two to three minutes.

Breeding

Breeding is moderately difficult and requires a dedicated setup. Unlike many other cyprinids that scatter eggs randomly, Trigonostigma species lay their eggs on the underside of broad leaves. A breeding tank should have very soft, acidic water (pH 5.0-6.0) and plenty of broad-leaved plants like Cryptocoryne. After a successful spawning where the male wraps around the female to fertilize the eggs, the parents must be removed immediately to prevent them from eating the eggs. The fry will hatch in about 24 hours and require infusoria until they are large enough to eat baby brine shrimp.

Conclusion

The Truncate Harlequin Rasbora is a magnificent, easy-to-care-for species that adds a brilliant splash of color and dynamic schooling behavior to any freshwater aquarium. Provided with clean water, a proper diet, and a peaceful environment, these resilient little fish will thrive and bring joy to their keepers for years to come.

Fish info

Care level: Easy
Temperament: Peaceful
Adult size: 3.5 - 4 cm
Life expectancy: 3-5 years
Fish type: Cyprinid
Temperature : 22-28°C
pH: 5.0-7.0
Color Form: Black, Clear, Gold, Orange, Silver

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