Harlequin Rasbora - Blue - Trigonostigma heteromorpha var. Blue Fish Profile & Care Guide
Introduction
The Blue Harlequin Rasbora (Trigonostigma heteromorpha var. Blue) is a selectively bred colour form of Trigonostigma heteromorpha rather than a separate species. Trade listings use the “Blue” name for this fish, while authoritative taxonomy still recognises the valid species as Trigonostigma heteromorpha (formerly Rasbora heteromorpha). Aquarium wholesalers also note that the blue and gold forms are colour sports and behave like the standard Harlequin. These rasboras are an excellent choice for aquarists who want both striking colour and active, engaging fish. Valued for their peaceful temperament and hardy nature, they settle well into community aquariums and are also well-suited to carefully planned nano aquascapes.
In captivity, Blue Harlequin Rasboras are naturally shoaling fish and do best when kept in groups of at least six. Watching their coordinated swimming and calm group behaviour adds movement and visual appeal to the aquarium, while also allowing their natural social instincts to be seen more clearly. For tankmates, the Blue Harlequin Rasbora is an excellent choice for a peaceful community setup, provided companions are similarly calm and not large enough to intimidate or outcompete it. Harlequins are shoaling fish and should be kept in groups, which improves confidence and creates more natural behaviour. In practice, that means pairing them with other small, non-aggressive species and avoiding boisterous or predatory fish that can stress a shoal, especially in smaller aquaria.
Aquarium requirements for Blue Harlequin Rasboras are the same as those for standard T. heteromorpha. As this species is associated with forest streams and peat-swamp influenced habitats, it tends to show its best colour and most natural behaviour in a well-maintained planted aquarium with subdued lighting, sheltered areas, and open swimming space for the shoal. Soft, slightly acidic water helps replicate its natural environment, while stable temperatures in the low- to mid-tropical range and consistently clean water are far more important than chasing extreme parameters.
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Sexual Dimorphism
Sexual dimorphism in Blue Harlequin Rasboras is present but not dramatic, and it is usually easier to judge in a settled group than in newly purchased fish. Females are typically fuller-bodied, especially when mature and carrying eggs, while males tend to be slimmer and often appear slightly more intensely coloured. In standard Harlequins, the shape of the dark wedge can also help, with males often showing a more rounded wedge edge and females a more angular appearance. However, this should be treated as a guide rather than a guarantee of a single feature.
Quick Facts
| Scientific Name | Trigonostigma heteromorpha var. Blue |
|---|---|
| Kingdom | Animalia |
| Classification | Actinopterygii |
| Order | Cypriniformes |
| Family | Cyprinidae |
| Genus | Trigonostigma |
| Origins | Malaysia Singapore Indonesia |
| Approx Max Size | 5 cm |
| Temperament | Peaceful |
| Aquarium Level | Middle - Top |
| Difficulty | Beginner - Intermediate |
| Shoaling | Yes |
| Best kept as | Groups 6+ |
| Diet & Feeding | Omnivore |
| Reproduction | Egg Depositor |
| Average Lifespan | Up to 8 Years |
Water Parameters
| Water Type | Freshwater |
|---|---|
| pH | 6.0 - 7.0 |
| GH | 2 - 15 |
| TDS | 18 - 215 |
| ℉ | 72 - 82 |
|---|---|
| ℃ | 22 - 28 |
Natural Habitat
For the Blue Harlequin Rasbora, a captive colour form of Trigonostigma heteromorpha, the natural distribution is the same as that of the standard species. Trigonostigma heteromorpha is native to parts of Southeast Asia, including the Malay Peninsula, Peninsular Malaysia, Singapore, and Indonesia. In the wild, this species is associated with shaded forest streams and peat-swamp influenced waters, where conditions are typically calm or gently flowing, soft, and acidic, with leaf litter and dense marginal cover. These habitats are usually dimly lit and rich in natural structure, which helps the fish feel secure.
Breeding
The Blue Harlequin Rasbora breeds in the same manner as the standard Trigonostigma heteromorpha, but its spawning strategy differs from many other small cyprinids. Instead of scattering eggs at random, it usually deposits them on the underside of broad leaves or other smooth surfaces, so it is more accurately described as a leaf-spawning egg layer. When breeding is attempted in captivity, success is usually enhanced by providing very soft, slightly acidic water and suitable broad-leaved plants to encourage natural spawning behaviour.
Diet & Feeding
In nature, the Blue Harlequin Rasbora feeds on worms, crustaceans, and insects, which supports offering a varied, protein-rich diet in captivity. In the aquarium, they do best on a quality small flake or micro-pellet staple, supplemented regularly with appropriately sized live or frozen foods such as daphnia, baby brine shrimp, and small bloodworm to maintain condition and colour. Feed little and often, making sure food is small enough for the shoal to take comfortably and that excess is not left to foul the water.
Other Rasboras
Published: 24 Feb 2026
Last Updated: 24 Feb 2026