Blackline rasbora

The blackline rasbora (Rasbora borapetensis), also known as the brilliant rasbora or red-tailed rasbora, is a lively schooling fish native to the slow-moving streams and swamp forests of Southeast Asia, particularly the Mekong, Chao Phraya, and Mae Klong basins of Thailand, Cambodia, Laos, and Vietnam. It is named for the bold black horizontal stripe running from its gill cover to the base of its tail, flanked by a narrow golden line above. Its most striking feature is the bright red base of the caudal fin, which contrasts beautifully with its sleek silver body — a combination that makes it instantly recognizable among the smaller rasboras.

In the aquarium, blackline rasboras are exceptionally peaceful and spend most of their time darting through the middle and upper water levels in coordinated schools. They thrive in groups of eight or more, where males display intensified coloration as they compete for attention. Ideal tankmates include other small cyprinids, tetras, danios, and bottom-dwelling Corydoras catfish. Avoid large or aggressive species that might intimidate them. In the wild they are micropredators, sifting through leaf litter for small insects, crustaceans, and zooplankton; in captivity they accept quality flake foods but show their best colour and condition when offered regular live or frozen foods such as daphnia, brine shrimp, and bloodworms.

A minimum tank of 70 litres (about 18 US gallons) with soft, slightly acidic to neutral water and gentle filtration suits them well. A dark substrate, dense planting, and floating plants to diffuse overhead light bring out their richest colours. They are hardy and adaptable — most specimens available today are commercially farmed — but they do best with stable water parameters and regular partial water changes. Like many rasboras, they are egg-scatterers with no parental care. Eggs hatch within 24 to 48 hours, and the tiny fry become free-swimming a day or two later, accepting infusoria and later brine shrimp nauplii.