FishfreshwaterUndemanding

Topmouth gudgeon

A small unremarkable-looking cyprinid that is among the most damaging freshwater invaders in Europe, principally because it carries a parasite lethal to other fish while remaining healthy itself. It is listed under European Union invasive-species law and is included here so that it can be recognised rather than kept.

Quick profile

Scientific name: Pseudorasbora parva
Adult size: 6–9 cm (2.4–3.5 in)
Lifespan: 3–5 years
Temperature: 2–30 °C (36–86 °F)
pH: 6.5–8.5
Hardness: 4–25 °dGH
Minimum tank: 60 L (16 US gal)
Minimum length: 60 cm
Swimming zone: All levels
Social: Loose group
Diet: Omnivore
Overall confidence: moderate confidence

What keeping this species involves

Physically it will live in almost anything, which is exactly the problem. The reason not to keep it is legal and ecological rather than practical.

Slow lowland waters of almost any description: ponds, ditches, canals, reservoir margins and river backwaters.

Identity and names

Common name: Topmouth gudgeon
Scientific name: Pseudorasbora parva
Family: Cyprinidae
Order: Cypriniformes
Class: Actinopterygii
Also sold as: Pseudorasbora parva, stone moroko, false harlequin

Adult size and growth

6–9 cm (2.4–3.5 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

Recorded for completeness. This is a species AquariumHQ does not recommend keeping, and where it is prohibited the question of tank size does not arise.

Temperature (°C)

Topmouth gudgeon

2–30

Scale: 0 to 36 °C.

pH (pH)

Topmouth gudgeon

6.5–8.5

Scale: 4 to 9.5 pH.

Hardness (°dGH)

Topmouth gudgeon

4–25

Scale: 0 to 30 °dGH.

The bands AquariumHQ publishes for the topmouth gudgeon, each drawn against the full range this catalogue uses for that parameter. A narrow band on a wide scale is a specialist; a wide one is a genuinely tolerant animal. Source: AquariumHQ husbandry position for this record — an editorial range, not a measurement

Why these ranges: The tolerance recorded for this species spans nearly everything a temperate fresh water can do, which is a large part of why it establishes so readily. The figures describe its documented range rather than a husbandry recommendation.

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	2–30 °C (36–86 °F)	Stability matters more than hitting an exact figure.
pH	6.5–8.5	A stable pH outside the ideal band is usually better than an unstable one inside it.
General hardness	4–25 °dGH	Check your supplier's published figure before choosing species.
Minimum volume	60 L (16 US gal)	A floor, not a recommendation.
Minimum footprint	60 cm long × 30 cm deep	Recorded for completeness. This is a species AquariumHQ does not recommend keeping, and where it is prohibited the question of tank size does not arise.
Water flow	low	Match the outflow to this rather than to the filter's rating.
Lid required	Recommended	Reduces evaporation and prevents accidents.
Structure	Vegetation and hard surfaces for the male to guard eggs on.

Behaviour

Feeds opportunistically through the water column and takes eggs and larvae of other species where it finds them.
Males guard a cleaned surface and drive off fish considerably larger than themselves.

Telling males from females

Breeding males are much darker with prominent tubercles on the snout and are larger than females.

Feeding

Eats zooplankton, invertebrates, algae and the eggs and fry of other fish. The last of those is part of why it displaces native species.

Diet type: Omnivore
Staples: Quality flake, Frozen daphnia, Frozen bloodworm

Breeding

Strategy: Eggs attached to a hard surface and guarded by the male
Difficulty: incidental
Parental care: guarding

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.

Risk this species poses to common tankmates
Towards	Risk	What it means
Small fish	moderate	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	high	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	occasional	Fin damage is an entry point for secondary infection.
Plants	none	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.

A healthy carrier of the rosette agent, an intracellular parasite that causes severe disease and mortality in other cyprinids while leaving this species largely unaffected. That is the main mechanism behind its impact on native fish.
Arrived in Europe as a contaminant of carp stocking consignments rather than through the aquarium trade, and spread from there. It is included in this catalogue as a species to identify, not as one to acquire.

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: East Asia natively — China, Japan, Korea and the Amur basin; invasive across Europe and beyond
Habitat: Slow lowland waters of almost any description: ponds, ditches, canals, reservoir margins and river backwaters.

What the checklists say

77 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	1	A checklist states the species occurs here naturally.
Introduced	61	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	14	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

Listed as an invasive alien species of Union concern under EU Regulation 1143/2014, which prohibits keeping, transport, sale, breeding and release across the European Union. Great Britain applies equivalent restrictions under its own retained regime. These are the two jurisdictions AquariumHQ has checked; other countries have their own rules and this listing should not be generalised to them.

Where this species has been introduced, and what followed

Europe — recorded in 40 countries, first in the Czech Republic in 1960

Established

Arrived as a contaminant in consignments of farmed Asian carp in the 1960s and spread across forty countries in sixty years. EASIN flags the angling pathway HIGH uncertainty, intentional release LOW, and lists the aquarium trade unflagged. The dominant mechanism is that nobody meant to move it at all.

Escape from food or bait aquacultureFood and bait production, at a scale no private keeper approaches. The animals are held in numbers, in ponds, often on floodplains.
Deliberate stockingSomebody put the species into open water on purpose, for food, angling, ornament or mosquito control — frequently a government agency, and frequently before anyone considered the consequences.
Deliberate aquarium releaseSomebody took the animal out of a tank and put it into open water. The commonest reason is that the fish outgrew the aquarium, which is why size at maturity is a conservation question as well as a welfare one.

What has been established

Carries parasites or pathogensMeasured in studyRated HIGH against animal health and HIGH against aquaculture. The topmouth gudgeon is a healthy carrier of Sphaerothecum destruens, the rosette agent — an intracellular parasite that is lethal to several native European cyprinids and to salmonids. This is the reason it is treated as one of Europe's most serious freshwater invaders, and it is a disease mechanism rather than a competitive one.
Competition with native speciesMeasured in studyRated HIGH against nature protection and restoration and MEDIUM against fisheries.

Of Union concern. Not an aquarium fish, and included because it is the best European example of an invasion whose damage is done by something the animal carries rather than by the animal.

EASIN — European Alien Species Information Network · EASIN catalogue record R12451 (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.

What AquariumHQ has recorded, by jurisdiction, with the instrument each entry comes from and how far the check went. Ecological risk and legal status are separate: a species can be ecologically risky and legally unrestricted, or restricted where it could never establish.

EU

Prohibitedpossession

Pseudorasbora parva Temminck & Schlegel, 1846 appears by name in the base Union list. Article 7 prohibits keeping, breeding, transporting, placing on the market, using, exchanging and releasing it anywhere in the Union.

Instrument: Regulation (EU) No 1143/2014, Article 7, as applied to the Union list in Commission Implementing Regulation (EU) 2016/1141 (consolidated text 02016R1141-20220802)

Checked against the instrument · last checked 2026-08-29 · An editor has read the current text of the named instrument and confirmed this species' entry on the date shown.

US

Prohibitedimport

The species is listed by name as stone moroko among the injurious Cyprinidae. The prohibition is on importation, transportation and acquisition of live fish, gametes, viable eggs or hybrids; the operative words in the section are 'importation, transportation, or acquisition', and possession as such is not addressed by it.

Instrument: 50 CFR § 16.13(a)(2)(v)(H), injurious wildlife listing under 18 U.S.C. 42

Two independent jurisdictions reached the same conclusion about this fish through different routes and for the same reason: it is a healthy carrier of the rosette agent, which is lethal to other cyprinids. This is the clearest dual listing in the catalogue.

Checked against the instrument · last checked 2026-08-29 · An editor has read the current text of the named instrument and confirmed this species' entry on the date shown.

GB

Prohibitedrelease

Release or escape of any non-native animal is an offence, and this species arrived in Britain as a contaminant of stocking consignments rather than by deliberate release.

Instrument: Wildlife and Countryside Act 1981, section 14

Instrument recorded, species entry not re-checked · last checked 2026-08-11 · AquariumHQ has recorded the instrument and what it does, but has not re-confirmed this species' entry against the current consolidated text. Listings are amended, so treat this as a pointer to the right law rather than as a confirmation of the current position, and check with the authority before acting.

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	Pseudorasbora parva	Observed / measured	high confidence	Catalogue of Life; GBIF — Global Biodiversity Information Facility
native range	East Asia natively — China, Japan, Korea and the Amur basin; invasive across Europe and beyond	Observed / measured	moderate confidence	GBIF — Global Biodiversity Information Facility
adult size	6–9 cm	Aquarium-literature consensus	moderate confidence	AquariumHQ husbandry synthesis
lifespan	3–5 years	Aquarium-literature consensus	moderate confidence	AquariumHQ husbandry synthesis
temperature	2–30 °C	Aquarium-literature consensus	moderate confidence	AquariumHQ husbandry synthesis
ph	6.5–8.5 pH	Aquarium-literature consensus	moderate confidence	AquariumHQ husbandry synthesis
general hardness	4–25 °dGH	Aquarium-literature consensus	moderate confidence	AquariumHQ husbandry synthesis
minimum tank volume	≥ 60 L	AquariumHQ editorial position	moderate confidence	AquariumHQ husbandry synthesis
minimum tank length	≥ 60 cm	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.