Slender Bitterling

The slender bitterling, also known as the Taiwanese bitterling, is a temperate freshwater cyprinid native to Taiwan and parts of eastern China, including Jiangsu, Anhui, and Fujian provinces. It reaches about 6–8 cm in length and displays a silvery body with a subtle blue-green iridescence along the flank, highlighted by a dark lateral stripe that becomes more pronounced during the breeding season. Males develop brighter colours — particularly red-orange on the fins and body — when in spawning condition. The species has been classified under multiple scientific names including Paratanakia himantegus and Acheilognathus himantegus, reflecting ongoing taxonomic refinement within the bitterling group.

As a subtropical species, the slender bitterling is more tolerant of cooler temperatures than most aquarium fish. They thrive in a range of 15–28°C, making them suitable for unheated indoor tanks in temperate climates, though stable conditions are still important. A minimum of 80 litres is recommended for a small group, with larger being better for a proper school of six or more. The aquarium should have moderate to strong water flow with plenty of oxygenation — a powerhead is beneficial. Décor can include a gravel or sand substrate, smooth rocks, and driftwood branches, with robust plants that can tolerate the flow. These fish are known jumpers, so a secure lid is necessary. Water should be neutral to slightly alkaline with a pH of 6.5–7.8.

Slender bitterlings are peaceful, active shoalers that mix well with other temperate community fish of similar size — danios, minnows, barbs, and Stiphodon gobies are all suitable companions. They are omnivorous and accept most aquarium foods: quality flake, micro-pellets, and live or frozen daphnia, brine shrimp, bloodworms, and mosquito larvae. They also browse on algae. A fascinating aspect of their natural history is their breeding strategy: bitterlings are obligate mussel-spawners, with females using a long ovipositor to deposit eggs into the mantle cavity of live freshwater mussels, where the eggs develop safely. This makes home breeding challenging, though possible with appropriate unionid mussels. With proper care, they live four to six years.