Exclamation Point Rasbora (Boraras urophthalmoides)

The Exclamation Point Rasbora (Boraras urophthalmoides) is a micro-sized cyprinid from Southeast Asia, distributed across Thailand, Cambodia, Laos, and Vietnam where it inhabits slow-moving peat swamp streams, forest pools, and blackwater rivers with soft, acidic, heavily stained water and dense aquatic vegetation.

Reaching only 1.5–2 cm in length, B. urophthalmoides is among the smallest aquarium fish available. Its common name derives from the characteristic black marking pattern on the flank and caudal peduncle that, when viewed together, resembles an exclamation point (!). The base coloration varies from pale translucent pinkish to deeper rose-red depending on condition and diet, with males tending to be more intensely colored.

As a member of the genus Boraras, it is closely related to other popular nano rasboras including the Chili Rasbora (B. brigittae) and Dwarf Rasbora (B. maculatus). Like its congeners, it is a blackwater specialist that benefits greatly from soft, acidic, tannin-rich water in the aquarium.

Exclamation Point Rasboras are active, midwater schooling fish that look most impressive in large groups of 20 or more. Their tiny size and peaceful disposition make them compatible with freshwater shrimp colonies, making them a popular choice for shrimp-focused planted tanks.

Exclamation Point Rasboras require soft, acidic, blackwater-style conditions: pH 4.0–7.0, hardness 1–5 dGH, and temperatures 22–28°C. While they survive in slightly harder water, the most vibrant coloration and active behavior are observed in soft, tannin-rich water that mimics their natural habitat.

A nano aquarium of 30–40 liters is suitable for a group of 15–20 fish. Create a blackwater environment with a dark substrate, driftwood, dried botanicals, and Indian almond leaves. Dense plantings of fine-leaved plants (Java moss, Rotala, Riccardia) provide cover and spawning sites. Dim lighting or floating plants to reduce light intensity is preferred.

Feed the smallest available foods: infusoria, micro worms, baby brine shrimp nauplii, daphnia, and micro pellets. Their mouths are extremely small — many standard aquarium foods may be too large. Feed small amounts 2–3 times daily.

Exclamation Point Rasboras are entirely peaceful and safe with all similarly-sized species and freshwater shrimp (Neocaridina, Caridina). Avoid any fish large enough to swallow them — even small tetras (3 cm+) may consume them. Ideal tankmates are other Boraras species, small Corydoras, and peaceful nano fish.

Maintain groups of at least 15–20 for natural behavior and stress reduction. Regular small water changes (15–20% twice weekly) maintain water quality without causing parameter shock.