Least rasbora

The least rasbora or exclamation-point rasbora (Boraras urophthalmoides) is one of the smallest freshwater aquarium fish in the world, reaching a maximum size of just 2 cm (0.8 in). It is native to shallow, densely vegetated marshes, peat swamps, rice paddies, and slow-moving ponds across Thailand, Cambodia, and Vietnam. The common name "exclamation-point rasbora" refers to the prominent dark spot on the caudal peduncle, which is elongated into a horizontal dash — resembling an exclamation mark when combined with the dark midlateral stripe. Its tiny size and brilliant red–orange colouration when in peak condition make it a sought-after nano-species.

This species requires soft, acidic, tannin-stained water typical of Southeast Asian blackwater habitats: pH 6.0–7.0, hardness 2–10 dGH, and temperature 24–28°C. It is sensitive to water quality and parameter swings, so a mature, stable aquarium is essential. A tank of at least 20 L (5 gal) will house a small group, but 40 L (10 gal) or more is strongly recommended for a proper shoal. Dense planting, floating plants to diffuse light, a dark substrate, and gentle sponge filtration replicate their natural environment best.

Least rasboras are extremely peaceful but timid, and must be kept in groups of at least 10–12 to feel secure and display natural behaviour. They are micropredators with tiny mouths — offer infusoria, vinegar eels, microworms, baby brine shrimp, and finely crushed flake or micro-pellets. They will not compete well with larger or more boisterous tankmates. Ideal companions include chili rasboras, pygmy corydoras, dwarf shrimp, and other nano-species. Their small bioload makes them suitable for heavily planted nano-tanks. With stable conditions, lifespan is 3–5 years.