Bitterling

The European Bitterling is a small, iridescent fish found across much of Europe and Central Asia in slow-moving or still waters containing freshwater mussels. It is the female's use of a long ovipositor to deposit eggs inside living mussels for incubation that sets this species apart from virtually all other aquarium fish.

Males in breeding condition develop iridescent blue-green flanks, rosy belly, and vivid fin colouration, making them genuinely striking. Outside breeding season both sexes are subtly silvery with a metallic blue-green sheen on the flanks.

To observe the complete breeding behaviour a large unionid mussel must be present in the aquarium. Providing clean, cool, oxygen-rich water and a mussel host is the main challenge of keeping this species, elevating it to intermediate difficulty.