Rose bitterling

The rose bitterling (Rhodeus ocellatus) is a small, peaceful East Asian fish with one of the strangest breeding habits in fresh water. For most of the year it is an unassuming silvery fish up to 9.2 cm standard length. Then, from spring into summer, breeding males flush a deep rosy red, sometimes purple, to court females, which is where the name comes from. It is a subtropical, cool-water species reported from eastern Asia, including Taiwan and Russia; an unheated tank is suitable only where room temperatures meet its seasonal requirements, comfortable somewhere around 18-24 °C at pH 7.0–7.5 in moderately hard water; with a cooler winter rest. Feed it as the omnivore it is, on algae, soft plant matter, and small live or frozen foods.

The remarkable part is how it breeds, because it cannot do so without a partner of a completely different kind: a living freshwater mussel. A breeding female uses an egg-laying tube to deposit eggs inside a living mussel; the male fertilises them there, and the young remain in the bivalve until they can swim. Keeping the mussel alive is itself demanding, so breeding should be attempted only with a suitable mussel-care plan.

So if you want to watch the full life cycle, you need healthy mussels in the tank, which is a real challenge of its own. Without them you can still enjoy a hardy, active fish that colors up beautifully at spawning time; you just won't get fry.

Provide a cool winter rest, circulation and good oxygenation.