At a glance the quick version
Looks how to spot one
Small, deep-bodied scaled fish
A small, compact fish. In breeding season the female grows a long egg tube (ovipositor) that she uses to place eggs inside a freshwater mussel.21
Grooming and care the hands-on bits
To see natural breeding, the pond or tank needs living freshwater mussels as well as plants and a soft, sandy bottom.21
Home where they are happiest
Still or slow-moving cool water with thick plant growth and a sandy or silty bottom; a temperate coldwater fish.1
Food basics what goes in the bowl
Mainly plant matter, topped up with worms, small crustaceans and insect larvae.12
The story where it all began
The bitterling is a native fish of European lowland waters whose life is tied to freshwater mussels. In the mid 20th century it gained an unusual job as a living pregnancy test, and today it is kept in the aquarium and pond hobby.21
Fun facts for the dinner table
- 1
The female lays her eggs inside a living freshwater mussel, and the male releases sperm into the water the mussel breathes in, so fertilisation happens inside the shell.2
- 2
The young stay safe inside the mussel for about a month before swimming out.2
- 3
Bitterlings were once used as a pregnancy test: hormones in a pregnant woman's urine made the female fish's egg tube extend.2
- 4
They are grown up after just one year, at only 3 to 3.5 cm long.2
Recognised by the official bits
- IUCN Red List
- Assessed as Least Concern (2022)1
The care rhythm Furry Diary remembers it for you
A typical home-care rhythm for a European Bitterling. It is our own starting plan, not a vet's advice, and you can change every line in the app.
- Feedingevery day
- Water changeAbout 25 to 30% of the waterevery week
- Water testevery week
- Glass cleanevery week
- Filter rinseevery month
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Where this comes from
Every breed page shows where each fact comes from. If we got something wrong, sorry, we are pet people, not a science magazine. Tell us at [email protected] and we will fix it.
- FishBase: Rhodeus amarus fishbase.seused for range, size, age, habitat, diet, spawning, IUCN status
- Wikipedia: European bitterling en.wikipedia.orgused for range, size, maturity, mussel breeding, pregnancy test history, fun facts
Checked 2026-09-30. Text is written in our own words from these sources.