What tank size does a Molly need?

A Molly needs an aquarium of at least 90 litres (23.8 US gallons) with a footprint at least 75 cm (29.5 in) long — for this species, swimming length matters as much as raw volume.

What water parameters does a Molly need?

Molly does best at 24–28 °C (75.2–82.4 °F), pH 7.5–8.5 and a general hardness of 15–30 °dGH.

What group size does a Molly need?

Molly is a social species — keep a group of at least 6 so natural behaviour emerges and stress stays low.

Is the Molly suitable for beginners?

Yes — Molly is one of the easier species to keep and forgives typical first-tank mistakes, as long as the aquarium is cycled and maintained.

Species library

Molly

Poecilia sphenops

Poeciliidae

FishDifficulty: ●●○○○Data: well-established

📷 Dnoerholm · CC BY-SA 4.0 · resized · Wikimedia Commons

Molly — Poecilia sphenops — 📷 Dnoerholm · CC BY-SA 4.0 · resized · Wikimedia Commons

About

The common or short-finned molly is a sizeable, adaptable livebearer native to the coastal fresh and brackish waters of Central America. Wild fish are silvery-olive, but aquarium strains include the ubiquitous jet-black molly, dalmatian, gold and lyretail forms. Mollies are hardy but appreciate warm, hard, alkaline water and are far more robust with a little added salt or mineral hardness; soft acidic water leads to disease and finger-fungus. Males can be pushy toward each other, so keep a female-skewed group in a spacious tank.

Care

Keep in warm, hard, alkaline water 24-28°C with strong filtration and a mostly herbivorous diet. Favour more females than males; benefits from added mineral hardness.

Diet

Herbivore-leaning omnivore; algae, spirulina and blanched veg are essential, with modest protein.

Tankmate compatibility

Computed by the AquaSage engine for a spacious reference tank — always check the full verdict for your exact setup in the calculator.

Full tank-mates guide with reasons →

Telling males from females

What works for this species, most reliable first.

The gonopodiumDefinitive · from about 4 weeks old
Look at the anal fin, underneath and just behind the belly. In females it stays a broad triangular fan. In males it narrows into a rod and angles forward — that rod is the gonopodium, a rebuilt fin rather than an organ that appears from nowhere.

All the methods, with diagrams

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Frequently asked questions

What tank size does a Molly need?: A Molly needs an aquarium of at least 90 litres (23.8 US gallons) with a footprint at least 75 cm (29.5 in) long — for this species, swimming length matters as much as raw volume.
What water parameters does a Molly need?: Molly does best at 24–28 °C (75.2–82.4 °F), pH 7.5–8.5 and a general hardness of 15–30 °dGH.
What group size does a Molly need?: Molly is a social species — keep a group of at least 6 so natural behaviour emerges and stress stays low.
Is the Molly suitable for beginners?: Yes — Molly is one of the easier species to keep and forgives typical first-tank mistakes, as long as the aquarium is cycled and maintained.

Sources

Water & tank

Temperature24–28°C

Optimal temp.25–27°C

pH7.5–8.5

Hardness (GH)15–30°d

Carbonate (KH)10–20°d

Adult size10 cm

Min. volume≥ 90 L

Min. tank length≥ 75 cm

Group size≥ 6

Swimming zoneMid

TemperamentPeaceful

Water typeBrackish-tolerant

FlowModerate

Lifespan~4 yrs

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