Four-eyed fish
A surface-dwelling estuarine livebearer whose eyes are divided horizontally into an aerial and an aquatic half, so it sees above and below the waterline simultaneously. It is one of the most distinctive fish in the trade and one of the most consistently kept wrongly: it is not a freshwater fish, it is not a community fish, and it needs surface area rather than depth.
Quick profile
- Scientific name
- Anableps anableps
- Adult size
- 20–30 cm (7.9–11.8 in)
- Lifespan
- 4–8 years
- Temperature
- 24–30 °C (75–86 °F)
- pH
- 7.5–8.5
- Hardness
- 12–30 °dGH
- Minimum tank
- 250 L (66 US gal)
- Minimum length
- 150 cm
- Swimming zone
- Surface
- Social
- Loose group — at least 4
- Diet
- Insectivore
- Overall confidence
- moderate confidence
What keeping this species involves
Needs a large, long, shallow brackish tank with a lid, live food, and tankmates that share both its salinity and its size. Almost nothing in a general aquarium shop meets that description.
Intertidal mudflats, mangrove creeks and estuarine shallows, where the fish lies at the surface with the upper half of each eye above the waterline.
Identity and names
- Common name
- Four-eyed fish
- Scientific name
- Anableps anableps
- Family
- Anablepidae
- Order
- Cyprinodontiformes
- Class
- Actinopterygii
- Also sold as
- Anableps anableps, cuatro ojos, largescale four-eyes
Adult size and growth
20–30 cm (7.9–11.8 in) — Females considerably larger than males, and 30 cm is reached in captivity.
Typical lifespan in an aquarium: 4–8 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: elongate with a moderate. Assigned from what is typical of Anablepidae, not examined for this species.
Water and tank requirements
A surface fish that swims in a straight line and needs a long shallow footprint. Depth is almost useless to it; surface area is everything, and a tall tank of the same volume is the wrong tank.
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: Estuarine mudflat and mangrove water. The hardness and pH follow the salinity rather than the other way round, and the salinity is a requirement: this species is sold as a freshwater fish and does not thrive as one. Brackish at around 1.005 to 1.015 specific gravity is where it should be kept, and AquariumHQ publishes no conversion tool for that figure because it cannot yet do so defensibly.
Where these figures come from
Habitat class
Brackish and estuarine — intertidal mudflats and mangrove creeks of the Guianas
Water where fresh meets salt — estuaries, mangrove, tidal creeks and coastal lagoons — with salinity that changes with the tide.
Salinity is a requirement, not a tolerance, for the species genuinely from here — and the commonest way these animals are killed is being sold and kept as freshwater fish.
- Conductivity
- 2000–35000 µS/cm
- Salinity
- 1–20 ppt
- Flow
- variable
- 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.
- 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 | 24–30 °C (75–86 °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 | 12–30 °dGH | Check your supplier's published figure before choosing species. |
| Minimum volume | 250 L (66 US gal) | A floor, not a recommendation. |
| Minimum footprint | 150 cm long × 50 cm deep | A surface fish that swims in a straight line and needs a long shallow footprint. Depth is almost useless to it; surface area is everything, and a tall tank of the same volume is the wrong tank. |
| Water flow | low | Match the outflow to this rather than to the filter's rating. |
| Lid required | Yes | This species jumps. |
| Structure | A long shallow water column, ideally with an exposed sand or mud bank the fish can rest against. Mangrove roots or branch structure at the surface. |
Behaviour
- Rests at the surface with the upper eye half out of water, watching for insects and for anything overhead.
- Skitters across the surface when startled and leaves the tank readily. A lid is not optional.
- Beaches itself deliberately on mud banks in the wild, which a tank with a sloping sand bank allows.
Telling males from females
Females larger and deeper; males have a tube-like gonopodium that is fixed to one side.
Feeding
Feeds at and above the surface. Sinking food is largely wasted on it, which is a stocking consideration as much as a feeding one.
- Diet type
- Insectivore
- Staples
- Floating pellets, Live and frozen insects — crickets, bloodworm, mysis, Small crustaceans
Breeding
- Strategy
- Livebearer with internal fertilisation and asymmetric genitalia — a male whose gonopodium bends right can only mate with a female whose opening faces left
- Difficulty
- rarely achieved
- Parental care
- bearer
Broods are small and the fry large. The handedness of the genitalia is real and is the reason a randomly assembled pair may be physically unable to breed.
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 | high | Anything that fits in the mouth is at risk once this species reaches adult size. |
| Adult dwarf shrimp | near-certain | Cover reduces losses but rarely prevents them where the risk is high. |
| Shrimplets | near-certain | 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. |
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.
- Sold as a freshwater oddity. Kept in fresh water it declines slowly, and the decline is usually attributed to something else.
- A tall tank is the wrong shape at any volume. This animal needs length and surface, and a 250-litre cube serves it worse than a 150-litre long.
- Nearly all stock is wild-caught from estuarine fisheries.
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
- Atlantic coast of northern South America — from Trinidad and Venezuela through the Guianas to the Amazon mouth
- Habitat
- Intertidal mudflats, mangrove creeks and estuarine shallows, where the fish lies at the surface with the upper half of each eye above the waterline.
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 | Anableps anableps | Observed / measured | high confidence | Catalogue of Life; GBIF — Global Biodiversity Information Facility |
| native range | Atlantic coast of northern South America — from Trinidad and Venezuela through the Guianas to the Amazon mouth | Observed / measured | moderate confidence | GBIF — Global Biodiversity Information Facility |
| adult size | 20–30 cm | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| lifespan | 4–8 years | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| temperature | 24–30 °C | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| ph | 7.5–8.5 pH | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| general hardness | 12–30 °dGH | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| minimum tank volume | ≥ 250 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 | ≥ 4 | 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 2026-08-30. 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 checkableLimited evidence — treat with caution.
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. No checklist distribution statement is held for this species. The range on the page is editorial until one is.
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 loose group, and the number it is kept in decides how it behaves.
- Predator and prey sizeIt takes fish that fit in its mouth, and its mouth grows with it.
- Wild conditions and aquarium recommendationsThe water band on this page is an AquariumHQ position with a stated reason, not a measurement of a river.
- Rehoming an animal you can no longer keepThis animal reaches 30 cm. Most of the difficult rehoming cases are fish sold small that were always going to become impossible.
Group size and social behaviour
Lives in loose surface aggregations and is nervous alone. Males pursue females persistently and a group needs more females than males.