Molly
A large livebearer that needs hard, alkaline, mineral-rich water and does poorly without it. Persistent shimmying and fungal-looking patches in mollies are almost always a water chemistry problem rather than an infection.
Quick profile
- Scientific name
- Poecilia sphenops
- Adult size
- 8–15 cm (3.1–5.9 in)
- Lifespan
- 3–5 years
- Temperature
- 24–28 °C (75–82 °F)
- pH
- 7.5–8.5
- Hardness
- 15–30 °dGH
- Minimum tank
- 150 L (40 US gal)
- Minimum length
- 100 cm
- Swimming zone
- Midwater
- Social
- Loose group — at least 4
- Diet
- Herbivore
- Overall confidence
- high confidence
What keeping this species involves
Hardy in genuinely hard alkaline water and repeatedly troubled in soft water — which is where most keepers put them.
Hard, alkaline, mineral-rich water including coastal brackish lagoons and estuaries.
Identity and names
- Common name
- Molly
- Scientific name
- Poecilia sphenops
- Family
- Poeciliidae
- Order
- Cyprinodontiformes
- Class
- Actinopterygii
- Also sold as
- sailfin molly, black molly, short-fin molly
Name ambiguity
'Molly' in trade covers Poecilia sphenops, P. latipinna, P. velifera and hybrids between them. Most black and balloon mollies are hybrid strains with no single wild counterpart, which is why care information for them varies so widely.
Adult size and growth
8–15 cm (3.1–5.9 in) — Sailfin strains reach 12–15 cm; short-fin domestic strains stay nearer 8 cm.
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 Poeciliidae, not examined for this species.
Water and tank requirements
Large, active and heavily built livebearers producing a substantial bioload, kept in groups.
Temperature (°C)
Scale: 0 to 36 °C.
pH (pH)
Scale: 4 to 9.5 pH.
Hardness (°dGH)
Scale: 0 to 30 °dGH.
Where these figures come from
Habitat class
Hard alkaline water
Water draining limestone, gypsum or volcanic mineral — Central American rivers, karst springs, hard-water lakes.
These species are usually easy in hard tap water and difficult in the soft, acidic water the hobby treats as the default. The commonest failure is the keeper softening water that did not need softening.
- Conductivity
- 300–900 µS/cm
- Flow
- moderate
- Dissolved oxygen
- moderate
- Seasonality
- moderate
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.
- TemperatureFollows the habitat this animal comes from
- Follows the water this animal is recorded from — hard alkaline water. Hard alkaline water generally runs 22–30°C, and the advised band of 24–28°C is consistent with it. This is AquariumHQ's reasoning from the habitat, not a measurement of the species. A source does publish a figure for this parameter, and it answers a different question — an environmental range or a laboratory tolerance rather than a keeping recommendation. It is held alongside this band rather than behind it.
- pHFollows the habitat this animal comes from
- Follows the water this animal is recorded from — hard alkaline water. Hard alkaline water generally runs 7.4–8.5, and the advised band of 7.5–8.5 is consistent with it. This is AquariumHQ's reasoning from the habitat, not a measurement of the species. A source does publish a figure for this parameter, and it answers a different question — an environmental range or a laboratory tolerance rather than a keeping recommendation. It is held alongside this band rather than behind it.
- General hardnessFollows the habitat this animal comes from
- Follows the water this animal is recorded from — hard alkaline water. Hard alkaline water generally runs 10–25 dGH, and the advised band of 15–30 dGH is consistent with it. This is AquariumHQ's reasoning from the habitat, not a measurement of the species. A source does publish a figure for this parameter, and it answers a different question — an environmental range or a laboratory tolerance rather than a keeping recommendation. It is held alongside this band rather than behind it.
Scale: 20.8 to 31.2 °C.
Scale: 7.2 to 8.7 pH.
Scale: 7 to 33 dGH.
| Requirement | Value | Notes |
|---|---|---|
| Temperature | 24–28 °C (75–82 °F) | Stability matters more than hitting an exact figure. |
| pH | 7.5–8.5 | A stable pH outside the ideal band is usually better than an unstable one inside it. |
| General hardness | 15–30 °dGH | Check your supplier's published figure before choosing species. |
| Minimum volume | 150 L (40 US gal) | A floor, not a recommendation. |
| Minimum footprint | 100 cm long | Large, active and heavily built livebearers producing a substantial bioload, kept in groups. |
| Water flow | low | Match the outflow to this rather than to the filter's rating. |
| Lid required | Recommended | Reduces evaporation and prevents accidents. |
Behaviour
- Grazes algae from every surface all day.
- Tolerates brackish water well; some populations are naturally estuarine.
Telling males from females
Males have a gonopodium and, in sailfin strains, a greatly enlarged dorsal fin.
Feeding
Substantially herbivorous. A diet of protein-heavy flake is a common cause of the digestive problems attributed to 'molly disease'.
- Diet type
- Herbivore
- Staples
- Vegetable-based dry foods, Blanched courgette and spinach, Algae and biofilm
Breeding
- Strategy
- Livebearing
- Difficulty
- incidental
- Parental care
- bearer
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 | none | 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.
- Balloon-body strains are selectively bred for a shortened, curved spine that compresses the internal organs. AquariumHQ does not recommend them.
- 'Shimmying' — rocking on the spot without moving forward — is a recognised sign of unsuitable water chemistry, usually water that is too soft or too cold.
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
- Mexico and Central America; P. latipinna along the south-eastern United States coast
- Habitat
- Hard, alkaline, mineral-rich water including coastal brackish lagoons and estuaries.
What the checklists say
34 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 | 20 | 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 | 13 | 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
Where this species has been introduced, and what followed
Europe — Slovakia from 1971, also Germany, Spain and Italy
Locally establishedEASIN lists other intentional release, timber trade and the transport of habitat material. The timber-trade entry for a livebearing fish is not credible on its face and is recorded as EASIN publishes it — an aggregated pathway assignment that has picked up something from a source further back. Flagging it is more useful than silently correcting it.
- 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.
- Arrived with a shipment of something elseThe organism was not the thing being shipped. Snails on aquarium plants, clams in shellfish consignments, apple snails in farmed crayfish.
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 R11899 (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 | Poecilia sphenops | Observed / measured | high confidence | Catalogue of Life; GBIF — Global Biodiversity Information Facility |
| native range | Mexico and Central America; P. latipinna along the south-eastern United States coast | Observed / measured | moderate confidence | GBIF — Global Biodiversity Information Facility |
| adult size | 8–15 cm | Aquarium-literature consensus | high confidence | AquariumHQ husbandry synthesis |
| lifespan | 3–5 years | Aquarium-literature consensus | high confidence | AquariumHQ husbandry synthesis |
| temperature | 24–28 °C | Aquarium-literature consensus | high confidence | AquariumHQ husbandry synthesis |
| ph | 7.5–8.5 pH | Aquarium-literature consensus | high confidence | AquariumHQ husbandry synthesis |
| general hardness | 15–30 °dGH | Aquarium-literature consensus | high confidence | AquariumHQ husbandry synthesis |
| minimum tank volume | ≥ 150 L | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
| minimum tank length | ≥ 100 cm | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
| minimum group size | ≥ 4 | AquariumHQ editorial position | high 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 2026-08-11. 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. 34 distribution statements held from GBIF checklists — native in 1, introduced in 19 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 sizeOne sourceBased on a verified authoritative source.1 citation, 1 independent origin
A single source: velazquez-2015-grijalva.
- 7.5 cm — FishBase · Maximum recorded · The range the species is recorded from · checked 2026-08-11
What this kind of claim needs: size a well-kept specimen reaches — no external source is required, provided AquariumHQ shows its reasoning. One authoritative source, which is what this kind of claim requires. Independent replication would be welcome and is not the standard.
What would improve this: Corroborate against a second, independent source.
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.
temperatureOne sourceAquariumHQ's own position, derived from evidenced inputs.1 citation, 1 independent origin
A single source: aquarien-atlas.
- 18–28 °C — FishBase · Tolerated range · The range the species is recorded from · checked 2026-08-11
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. Derived from the observed range rather than copied from it. The observed envelope spans conditions this species meets across seasons and localities; a keeper holds one temperature continuously. The advised band trims the lower end because sustained residence at an extreme is a different demand from encountering it, and stability matters more to a captive fish than breadth.
What would improve this: Corroborate against a second, independent source.
phOne sourceAquariumHQ's own position, derived from evidenced inputs.1 citation, 1 independent origin
A single source: fishbase.
- 7.5–8.2 pH — FishBase · Tolerated range · The range the species is recorded from · checked 2026-08-11
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. Derived from the observed range rather than copied from it. The advised band reaches past the observed envelope and carries no individual review. Commercially bred stock is generally more tolerant than wild records suggest, which is the usual explanation, and it has not been established for this species.
What would improve this: Corroborate against a second, independent source.
general hardnessOne sourceAquariumHQ's own position, derived from evidenced inputs.1 citation, 1 independent origin
A single source: fishbase.
- 11–30 °dGH — FishBase · Tolerated range · The range the species is recorded from · checked 2026-08-11
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. Derived from the observed range rather than copied from it. The advised band trims the lower end of the observed range. Hardness at the extremes of a wild record is generally tied to specific geology rather than to the fish's requirement, and captive-bred stock is raised in whatever water the farm has.
What would improve this: Corroborate against a second, independent source.
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 loose group, and the number it is kept in decides how it behaves.
- Scientific and trade namesThe trade name for this animal covers more than one species.
- The kinds of water aquarium fish come fromThis record's water bands follow hard alkaline water, which is the kind of water this animal is recorded from.
- 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
Males display to each other and to females almost continuously.