How big does Molly get?

Molly reaches an adult standard length of 5–10 cm (excluding the caudal fin). Size the tank for that, not for the juvenile you bring home — the difference is often several times over, and upgrading later costs more than buying once.

What water does Molly need?

24–28 °C, pH 7–8.4, 12–25 dGH, 8–20 dKH. That is only 4 °C wide, so stability matters more than hitting a particular number.

What is the smallest tank for Molly?

At least 60 L, and no shorter than 60 cm. That is the floor for long-term keeping, not for short-term survival: volume buys water stability and length buys swimming room, and neither substitutes for the other. It is a schooling fish: keep at least 3 together. Fewer than that and it hides, loses colour, and aggression has nowhere to spread.

What is Molly like to keep?

Temperament: peaceful. Difficulty: beginner. It occupies the middle and is omnivore. With plants it is "Nips new growth".

Can Molly be kept with its most common tank mate?

The checker returns "Can be kept together". Molly and Guppy is a well-established pairing. Individual fish still differ; the record says "usually fine", not "guaranteed".

G-01 / 03Freshwater

Molly玛丽鱼

Poecilia sphenops · Poeciliidae

A medium livebearer that loves hard water and grazes algae, but struggles in soft water.

Illustration to come

G-01 / 03 · Molly Poecilia sphenops

SpecificationsFish atlas

Name: 玛丽鱼
Scientific name: Poecilia sphenops
Family: Poeciliidae
Origin: Central America (Mexico to Colombia)
Adult size: 5–10 cm
Temperament: Peaceful
Diet: Omnivore
Water level: Middle
Level: Beginner
School size: ≥ 3
Min. tank: 60 L
Min. length: 60 cm
Plant safe: Nips new growth

Water requirementsFreshwater

Temp.24–28 °C

pH7–8.4

Hardness12–25 dGH

Alkalinity8–20 dKH

Units: temperature in °C, hardness in German degrees (dGH / dKH), marine salinity as specific gravity at 25 °C, volume in litres. These are the values recorded for the species and do not follow the site unit switcher.

Keeping notes

1Wants hard water and even tolerates light brackish conditions; in soft water it slowly clamps its fins and stops feeding.
2It grazes algae from glass and leaves, so keep the diet plant-heavy; all-protein feeding causes bloating.
3Adults reach 10 cm, so start at 60 L or more; cramped tanks build ammonia and bring on constant fin rot.
4Good with platies and swordtails, but it nips soft shoots, so expect damage to fine-leaved plants.

Check a tank mate

Jump straight to the verdict for Molly against this one — no need to pick it again.

Frequently asked questions

How big does Molly get?: Molly reaches an adult standard length of 5–10 cm (excluding the caudal fin). Size the tank for that, not for the juvenile you bring home — the difference is often several times over, and upgrading later costs more than buying once.
What water does Molly need?: 24–28 °C, pH 7–8.4, 12–25 dGH, 8–20 dKH. That is only 4 °C wide, so stability matters more than hitting a particular number.
What is the smallest tank for Molly?: At least 60 L, and no shorter than 60 cm. That is the floor for long-term keeping, not for short-term survival: volume buys water stability and length buys swimming room, and neither substitutes for the other. It is a schooling fish: keep at least 3 together. Fewer than that and it hides, loses colour, and aggression has nowhere to spread.
What is Molly like to keep?: Temperament: peaceful. Difficulty: beginner. It occupies the middle and is omnivore. With plants it is "Nips new growth".
Can Molly be kept with its most common tank mate?: The checker returns "Can be kept together". Molly and Guppy is a well-established pairing. Individual fish still differ; the record says "usually fine", not "guaranteed".