European bitterling
A small European cyprinid that lays its eggs inside living freshwater mussels through a long ovipositor, and whose larvae develop in the mussel's gill chamber. Keeping it as a breeding species means keeping the mussels too, which is the harder half of the arrangement.
Quick profile
- Scientific name
- Rhodeus amarus
- Adult size
- 5–9 cm (2–3.5 in)
- Lifespan
- 3–5 years
- Temperature
- 4–24 °C (39–75 °F)
- pH
- 7–8.5
- Hardness
- 8–25 °dGH
- Minimum tank
- 80 L (21 US gal)
- Minimum length
- 80 cm
- Swimming zone
- Midwater
- Social
- Shoaling — at least 6
- Diet
- Omnivore
- Overall confidence
- moderate confidence
What keeping this species involves
Easy to keep and difficult to breed, and the difficulty is entirely about the mussel. A cool unheated tank suits the fish and does not by itself suit a unionid.
Slow lowland rivers, canals, oxbows and ponds with soft bottoms, vegetation and — critically — living freshwater mussels.
Identity and names
- Common name
- European bitterling
- Scientific name
- Rhodeus amarus
- Family
- Cyprinidae
- Order
- Cypriniformes
- Class
- Actinopterygii
- Also sold as
- Rhodeus amarus, Rhodeus sericeus amarus, bitterling
- Former or synonymous names
- Rhodeus sericeus amarus
Adult size and growth
5–9 cm (2–3.5 in)
Typical lifespan in an aquarium: 3–5 years. Almost every animal sold in a shop is a juvenile, so the tank has to suit the size in the first line rather than the size in the bag.
Shape, which is what the compatibility engine reads instead of length alone: fusiform — the ordinary torpedo with a small terminal mouth. Assigned from what is typical of Cyprinidae, not examined for this species.
Water and tank requirements
A small shoaling fish that needs floor area for mussels and open water above them. The mussels rather than the fish set the substrate requirement.
Temperature (°C)
Scale: 0 to 36 °C.
pH (pH)
Scale: 4 to 9.5 pH.
Hardness (°dGH)
Scale: 0 to 30 °dGH.
Why these ranges: A temperate European lowland species that overwinters cold and spawns in spring warmth. The seasonal swing is part of its biology rather than a range of acceptable holding temperatures, and it will not spawn without the cool period.
Where these figures come from
Habitat class
Temperate still water
Ponds, lakes, canals and slow rivers outside the tropics, with a genuine winter.
The temperature range is the point. These animals experience a seasonal cycle, and keeping them at a constant tropical temperature shortens their lives without ever looking like the cause.
- Conductivity
- 150–800 µS/cm
- Flow
- still
- Dissolved oxygen
- variable
- Seasonality
- strong
These are figures for the KIND of water body, not measurements of this species. They are AquariumHQ’s summary of the general limnology and are given wide because the water is.
- TemperatureReasoned on this record
- This record states its own reason for this band. See the water section.
- pHReasoned on this record
- This record states its own reason for this band. See the water section.
- General hardnessReasoned on this record
- This record states its own reason for this band. See the water section.
| Requirement | Value | Notes |
|---|---|---|
| Temperature | 4–24 °C (39–75 °F) | Stability matters more than hitting an exact figure. |
| pH | 7–8.5 | A stable pH outside the ideal band is usually better than an unstable one inside it. |
| General hardness | 8–25 °dGH | Check your supplier's published figure before choosing species. |
| Minimum volume | 80 L (21 US gal) | A floor, not a recommendation. |
| Minimum footprint | 80 cm long × 35 cm deep | A small shoaling fish that needs floor area for mussels and open water above them. The mussels rather than the fish set the substrate requirement. |
| Water flow | low | Match the outflow to this rather than to the filter's rating. |
| Lid required | Recommended | Reduces evaporation and prevents accidents. |
| Structure | A deep bed of fine sand for mussels to burrow into, planting, and open water. Without living mussels the fish will live and will not breed. |
Behaviour
- Males guard the water immediately above a mussel and lead females to it, which is a territory defined by another animal's position.
- Females extend the ovipositor and insert it into the mussel's exhalant siphon to place eggs, a process taking only a few seconds.
Telling males from females
Breeding males are intensely coloured with pink and violet flanks and develop tubercles on the snout; females grow a long trailing ovipositor and are otherwise plain.
Feeding
Eats algae, plant material and small invertebrates. Vegetable matter forms a larger part of the diet than for most small cyprinids.
- Diet type
- Omnivore
- Staples
- Quality flake, Frozen daphnia, Spirulina flake, Frozen bloodworm
Breeding
- Strategy
- Eggs deposited inside a living freshwater mussel through the female's ovipositor
- Difficulty
- challenging
- Parental care
- none
The larvae develop in the mussel's gill chamber for around a month. The mussel is not a container but a living animal with its own demanding requirements, and keeping one alive in an aquarium is the difficult part.
Tankmates and compatibility
Assessments are derived from stored traits by the compatibility engine, with the reasoning shown. A verdict is a statement about probability, not a guarantee — individual temperament can override it.
| Towards | Risk | What it means |
|---|---|---|
| Small fish | none | Anything that fits in the mouth is at risk once this species reaches adult size. |
| Adult dwarf shrimp | none | Cover reduces losses but rarely prevents them where the risk is high. |
| Shrimplets | moderate | Determines whether a shrimp colony can grow rather than merely persist. |
| Snails | none | Applies to snails you wanted as well as snails you did not. |
| Long or trailing fins | none | Fin damage is an entry point for secondary infection. |
| Plants | grazes-algae | Tough species such as Anubias and Java fern survive most grazers. |
Any pair in the catalogue can be assessed with the compatibility checker.
Welfare considerations
Points where the way this species is commonly sold or kept differs from what it actually needs.
- The relationship is not one-sided: the mussel releases its own larvae onto the fish, which carry them until they drop off. Keeping the fish without mussels removes half of a two-way relationship.
- Introduced beyond its native range in parts of western Europe and Britain, where it is established. Release is prohibited in several of those jurisdictions.
Problems to watch for
Most problems in a tank containing this species are general rather than species-specific. Start with the troubleshooting index and work from the observation rather than from a guess at the cause.
What AquariumHQ does not yet hold
Species-level disease susceptibility is modelled in the graph and is not yet populated. Health pages will not be published until each condition has veterinary sourcing, because health content written without evidence is the most harmful thing this product could publish. The gap is recorded in the coverage report.
Where it comes from
- Native range
- Europe — from France and the Rhine east to the Black Sea basin, introduced further west
- Habitat
- Slow lowland rivers, canals, oxbows and ponds with soft bottoms, vegetation and — critically — living freshwater mussels.
What the checklists say
51 distribution statements retrieved from GBIF checklists. Establishment means is reproduced from the source where it states one, and left unstated where it does not — a record of presence is not a record of origin.
| Status | Places | What this means |
|---|---|---|
| Native | 3 | A checklist states the species occurs here naturally. |
| Introduced | 41 | A checklist states the species arrived here with human help. That is not a statement that it is established, harmful or illegal. |
| Recorded as invasive | 1 | A checklist states the species is invasive here. AquariumHQ reproduces that assessment; it does not make one. |
| Recorded as present, establishment means not stated | 6 | The source records the species as present and does not say whether it is native or introduced. AquariumHQ does not fill that in. |
Legal status and responsible practice
Restriction note
Non-native and established in parts of Britain and western Europe, where release into the wild is prohibited and keeping may be regulated. Native freshwater mussels are separately protected in many countries, which affects the legality of collecting the mussels this species needs to breed.
Where this species has been introduced, and what followed
Europe — recorded as introduced in 26 countries, first in the United Kingdom around 1900
EstablishedLive bait. Another European native translocated within Europe — and one whose larvae develop inside freshwater mussels, so moving bitterling and moving mussels are the same event twice over.
- Bait-bucket release by anglersAn angler empties an unused bait bucket at the end of a session. Documented as the primary route for several crayfish and small fish, and almost entirely absent from the aquarium hobby's account of the problem.
What has been established
Nothing. The species is present outside its native range and no study has measured what it does there. The absence of data is not the absence of effect — it means the question has not been asked, and this is the ordinary state of the evidence for most aquarium species that have established somewhere warm.
EASIN — European Alien Species Information Network · EASIN catalogue record R12961 (apixg/catxg) · read 2026-08-30. A European network: the routes and effects recorded here are the ones documented in Europe. EASIN compiles national inventories, DAISIE and CABI rather than measuring, and its pathway assignments carry its own uncertainty flags, which are reported above where it sets them.
Never release aquarium life into the wild
Aquarium animals, plants, and aquarium water must never be released into ponds, rivers, canals, drains or the sea — not even in a country where the species is sold freely. Released fish and invertebrates establish populations that displace native species, and released plants and water carry pathogens, parasites and fragments capable of spreading without any animal at all. If you can no longer keep an animal, rehome it through an aquarium society, a retailer that accepts returns, or another keeper.
- Rehome through a local aquarium society, a specialist retailer or another keeper.
- Unwanted plants go in household waste or a hot compost heap, never in a watercourse or a garden pond.
- Never empty a bucket of tank water into a drain that discharges to open water.
- If an animal is beyond rehoming, ask an aquatic veterinarian about humane euthanasia rather than releasing it.
Legal status is recorded from the sources named and was correct on the verification date shown. Rules change, and sub-national rules often differ from national ones. Check the current position with the relevant authority before acquiring, selling or moving any animal or plant.
Evidence
Every stored value with the kind of support behind it and where to check it. Evidence type and confidence are separate axes: an editorial position held confidently and a measurement held tentatively are different things.
| Claim | Value | Evidence type | Confidence | Source |
|---|---|---|---|---|
| scientific name | Rhodeus amarus | Observed / measured | high confidence | Catalogue of Life; GBIF — Global Biodiversity Information Facility |
| native range | Europe — from France and the Rhine east to the Black Sea basin, introduced further west | Observed / measured | moderate confidence | GBIF — Global Biodiversity Information Facility |
| adult size | 5–9 cm | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| lifespan | 3–5 years | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| temperature | 4–24 °C | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| ph | 7–8.5 pH | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| general hardness | 8–25 °dGH | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| minimum tank volume | ≥ 80 L | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
| minimum tank length | ≥ 80 cm | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
| minimum group size | ≥ 6 | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
Last reviewed 2026-08-10. Values labelled as an aquarium-literature consensus are AquariumHQ editorial positions rather than measurements — see how evidence is handled.
Why we say this
What each figure on this page rests on, and how far that is from being independently established.
Every figure on this page has a provenance. Open any claim to see what AquariumHQ holds for it, which sources say what, and what a claim of that kind actually needs. Different claims need different evidence: an accepted scientific name is settled by one authority, while a legal restriction needs a current primary instrument. A single source is often the right answer rather than a thin one.
scientific nameAquariumHQ's own position, externally checkableBased on a verified authoritative source.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: accepted scientific name and classification — 1 authoritative source in an appropriate context. Checked against gbif on 2026-08-29. One authoritative backbone is what this class requires.
What would improve this: Go and read a source. This is reachable and has not been reached.
native rangeAquariumHQ's own position, externally checkableBased on a verified authoritative source.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: where the species occurs — 1 authoritative source in an appropriate context. 51 distribution statements held from GBIF checklists — native in 3, introduced in 41, recorded invasive in 1 countries. Establishment means is reproduced from the source, never inferred from occurrence.
What would improve this: Go and read a source. This is reachable and has not been reached.
adult sizeAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: size a well-kept specimen reaches — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
lifespanAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
temperatureAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
phAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
general hardnessAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
minimum tank volumeAquariumHQ's judgement — no authority publishes thisAquariumHQ's own position, derived from evidenced inputs.
No external authority publishes this. The rationale carries the reasoning.
What this kind of claim needs: minimum tank size — no external source is required, provided AquariumHQ shows its reasoning. No authority publishes this. AquariumHQ derives it, and the derivation is recorded with its inputs and assumptions.
What would improve this: Keep the rationale current. There is no source to find.
minimum tank lengthAquariumHQ's judgement — no authority publishes thisAquariumHQ's own position, derived from evidenced inputs.
No external authority publishes this. The rationale carries the reasoning.
What this kind of claim needs: minimum tank size — no external source is required, provided AquariumHQ shows its reasoning. No authority publishes this. AquariumHQ derives it, and the derivation is recorded with its inputs and assumptions.
What would improve this: Keep the rationale current. There is no source to find.
minimum group sizeAquariumHQ's judgement — no authority publishes thisAquariumHQ's own position, derived from evidenced inputs.
No external authority publishes this. The rationale carries the reasoning.
What this kind of claim needs: group size — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Keep the rationale current. There is no source to find.
Guides relevant to this species
Chosen from this record's own traits rather than listed on every page.
- Schooling and shoalingThis species is recorded as shoal, and the number it is kept in decides how it behaves.
- Native, introduced and invasiveThis record carries an invasive or legal note, and those are five separate claims that are easy to read as one.
- Scientific and trade namesThis species has been published under more than one accepted name.
- Wild conditions and aquarium recommendationsThe water band on this page is an AquariumHQ position with a stated reason, not a measurement of a river.
- How aquarium species get into the wildThis species has a documented introduction outside its native range, with the route recorded.
Group size and social behaviour
Shoals loosely. Males become territorial around a chosen mussel in the spawning season and defend the water above it.