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.