Silver dollar
A disc-shaped shoaling characin the size of its name and considerably larger, closely related to the piranhas and almost entirely herbivorous. It is a peaceful fish that will eat every plant in an aquarium, which is the fact that decides where it can go.
Quick profile
- Scientific name
- Metynnis argenteus
- Adult size
- 12–15 cm (4.7–5.9 in)
- Lifespan
- 8–10 years
- Temperature
- 24–28 °C (75–82 °F)
- pH
- 5.5–7.2
- Hardness
- 2–12 °dGH
- Minimum tank
- 300 L (79 US gal)
- Minimum length
- 150 cm
- Swimming zone
- Midwater
- Social
- Shoaling — at least 5
- Diet
- Herbivore
- Overall confidence
- moderate confidence
What keeping this species involves
Hardy, peaceful and easy to feed, requiring a large tank, a shoal, and acceptance that a planted aquarium is not on offer.
Slow river reaches and flooded forest with overhanging vegetation, where the fish feeds on fallen plant material.
Identity and names
- Common name
- Silver dollar
- Scientific name
- Metynnis argenteus
- Family
- Serrasalmidae
- Order
- Characiformes
- Class
- Actinopterygii
- Also sold as
- Metynnis argenteus, silver dollar fish, metynnis
Adult size and growth
12–15 cm (4.7–5.9 in)
Typical lifespan in an aquarium: 8–10 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: disc — as deep as it is long with a moderate. A serrasalmid built as a disc. Its depth, not its length, is what stops a large cichlid taking it.
Water and tank requirements
A deep-bodied shoaling fish of fifteen centimetres that swims in open water. The shoal is the unit and the length is what it uses.
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: Tapajós clearwater and adjacent tributaries, soft and slightly acidic. The species is tolerant within that range and is more sensitive to nitrate accumulation than to the exact figures.
Where these figures come from
Habitat class
Clearwater lowland river
Transparent, mineral-poor rivers draining ancient shield geology — the Tapajós and Xingu type — with low sediment and good light penetration.
Soft water with more movement and more oxygen than a forest stream. Species from here generally want current and open swimming room as much as they want the chemistry.
- Conductivity
- 20–100 µS/cm
- Flow
- moderate
- Dissolved oxygen
- near saturation
- 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 | 24–28 °C (75–82 °F) | Stability matters more than hitting an exact figure. |
| pH | 5.5–7.2 | A stable pH outside the ideal band is usually better than an unstable one inside it. |
| General hardness | 2–12 °dGH | Check your supplier's published figure before choosing species. |
| Minimum volume | 300 L (79 US gal) | A floor, not a recommendation. |
| Minimum footprint | 150 cm long × 50 cm deep | A deep-bodied shoaling fish of fifteen centimetres that swims in open water. The shoal is the unit and the length is what it uses. |
| Water flow | moderate | Match the outflow to this rather than to the filter's rating. |
| Lid required | Yes | This species jumps. |
| Structure | Open swimming space with wood and rock. Plants will be eaten, so the choice is between artificial planting and none. |
Behaviour
- Shoals tightly and turns as a unit, which is what makes a group of them worth the tank they take.
- Startles hard and jumps, and a nervous shoal will strike the lid repeatedly at a sudden movement in the room.
Telling males from females
Males have a longer anal fin with a reddish edge and a more concave profile to it. Females are plainer and deeper.
Feeding
Almost entirely herbivorous, feeding on fallen leaves, fruit and seeds in the wild. A vegetable diet is a requirement and a protein-based one produces digestive problems.
- Diet type
- Herbivore
- Staples
- Blanched courgette, Blanched lettuce, Spirulina flake, Vegetable pellets
Breeding
- Strategy
- Eggs scattered among vegetation in soft acidic water and abandoned
- Difficulty
- challenging
- Parental care
- none
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 | 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 | none | Fin damage is an entry point for secondary infection. |
| Plants | consumes | Tough species such as Anubias and Java fern survive most grazers. |
Assessed pairings
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.
- Frequently sold to control plant growth or as an oddity. It reaches fifteen centimetres, needs a group, and its plant-eating is not selective.
- Nervous fish in an unshaded open tank injure their snouts against the glass. Floating cover and a settled shoal reduce this substantially.
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
- The Tapajós basin of Brazil and adjacent Amazon tributaries
- Habitat
- Slow river reaches and flooded forest with overhanging vegetation, where the fish feeds on fallen plant material.
What the checklists say
10 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 | 2 | A checklist states the species occurs here naturally. |
| Introduced | 1 | A checklist states the species arrived here with human help. That is not a statement that it is established, harmful or illegal. |
| Recorded as present, establishment means not stated | 7 | The source records the species as present and does not say whether it is native or introduced. AquariumHQ does not fill that in. |
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 | Metynnis argenteus | Observed / measured | high confidence | Catalogue of Life; GBIF — Global Biodiversity Information Facility |
| native range | The Tapajós basin of Brazil and adjacent Amazon tributaries | Observed / measured | moderate confidence | GBIF — Global Biodiversity Information Facility |
| adult size | 12–15 cm | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| lifespan | 8–10 years | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| temperature | 24–28 °C | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| ph | 5.5–7.2 pH | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| general hardness | 2–12 °dGH | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| minimum tank volume | ≥ 300 L | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
| minimum tank length | ≥ 150 cm | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
| minimum group size | ≥ 5 | 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. 10 distribution statements held from GBIF checklists — native in 2, introduced 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.
- Wild conditions and aquarium recommendationsThe water band on this page is an AquariumHQ position with a stated reason, not a measurement of a river.
Group size and social behaviour
A genuine shoaler that is nervous alone and crashes into things when startled. Five or more is where the fish settles.