The Harlequin Rasbora (Trigonostigma heteromorpha)

The Harlequin Rasbora (Trigonostigma heteromorpha) is a hardy, peaceful, and vibrant freshwater schooling fish that is highly popular for community aquariums. Known for their distinctive black triangular patch on the rear half of their bodies and striking reddish-copper coloration, they bring constant energy to the mid-water level of a tank. 

Overview & Quick Statistics

Adult Size: 1.5 to 2 inches (3.8 to 5 cm).Lifespan: 5 to 6 years with proper care.

Diet: Omnivore.Temperament: Exceptionally peaceful.

Minimum Tank Size: 10 gallons (38 liters), though 20 gallons (80 liters) is highly recommended for proper schooling behavior.

Water Parameters & Environment

Harlequin Rasboras originate from peat swamps and blackwater streams in Southeast Asia. While they prefer soft, slightly acidic water, captive-bred varieties are highly adaptable to standard tap water.

Temperature: 72°F to 82°F (22°C to 28°C).

pH Range: 6.0 to 7.5 (can tolerate down to 5.0 in specialized setups).

Water Hardness: 1 to 12 dGH.

Tank Setup: They thrive in heavily planted tanks with dark substrates and floating plants to mimic dim, natural lighting. Keep water flow low to moderate.

Social Behavior & Tank Mates

These are strict schooling fish. You must keep them in groups of 6 to 10 or more. Keeping them in tiny numbers will cause extreme stress, hiding, and faded colors.They are perfect neighbors for other non-aggressive fish.

Great Tank Mates: Small tetras, neon tetras, Corydoras catfish, dwarf loaches, dwarf gouramis, and peaceful shrimp.

Avoid: Large, predatory fish like Oscars or large cichlids that can easily swallow them.

Diet & Feeding

As omnivores, they are not picky eaters. Because they have tiny mouths, ensure their food is small enough to swallow easily.

Daily Foods: High-quality flake foods or micro-pellets.Treats: Live or frozen brine shrimp, Daphnia, or bloodworms to boost their red coloration and health.