Do mollies need salt in their water?

Not strictly, provided your tap water is sufficiently hard and alkaline. However, if your local water is soft, adding aquarium salt can help recreate their preferred mineral-rich environment and prevent health issues.

Why is my molly shaking and swimming in place?

This behaviour is called 'the shimmies'. It is an indicator of environmental stress rather than an infection, usually caused by water that is too cold, too acidic, or lacking in essential minerals (soft water).

How can I tell the difference between male and female mollies?

Males are generally smaller, slimmer, and feature a modified, pointed anal fin called a gonopodium used for mating. Females are larger, rounder, and have a standard, fan-shaped anal fin.

How many mollies should I keep together?

They are highly social and should be kept in groups of at least four. Always maintain a ratio of at least two females for every male to ensure the females aren't overly stressed by constant mating pursuits.

Why are my mollies dying in my community tank?

The most frequent cause is incorrect water chemistry. Mollies are often mistakenly kept alongside soft-water fish like tetras; in soft, acidic water, mollies' immune systems fail, leaving them vulnerable to disease.

FreshwaterTropicalBeginner

Common Molly

Poecilia sphenops

The Common Molly is a highly active, adaptable livebearer renowned for its myriad of colour morphs and preference for hard, alkaline water.

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Ideal Conditions

Temperature: 22 – 28 °C
pH range: 7 – 8.5
Hardness: 15 – 30 dGH
Salinity: 1 – 1.015 SG
Adult size: 6 – 12 cm
Standard 90L aquarium: 90 litres+
Lifespan: 3 – 5 years
Diet: Omnivore with a strong preference for vegetable matter and algae.
Water flow: Moderate
Preferred setup: Planted with hardy, salt-tolerant plants (if using salt) and ample open swimming space.

About the Common Molly

The Common Molly is one of the most ubiquitous and historically significant fish in the aquarium hobby. Revered for their vibrant, diverse colour morphs and active demeanour, they bring constant movement to the upper levels of the tank. While frequently sold as beginner fish, they have specific requirements regarding water chemistry that must be met to ensure their long-term wellbeing. In the right conditions, they are profoundly rewarding pets.

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Is it right for your tank?

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Where Common Molly comes from

Native range

Central America, South America

Countries / regions

Mexico, Guatemala, Colombia

Habitat

Highly adaptable, inhabiting shallow surface waters, coastal streams, swamps, and brackish estuaries.

Populations have been introduced worldwide due to the aquarium trade and mosquito control efforts.

Similar species

Sailfin Molly
Poecilia latipinna
Guppy
Poecilia reticulata
Endler's Livebearer
Poecilia wingei
Platy
Xiphophorus maculatus

We’re adding more full species guides as the catalogue grows.

Compatible tank mates

Reviewed guidance based on this species' recorded traits.

View all compatibility

Platy
Xiphophorus maculatus
Share the exact same preference for hard, alkaline water and have a similar peaceful but active temperament.
Great
Swordtail
Xiphophorus hellerii
Another robust livebearer that thrives in hard water environments alongside mollies.
Great
Bristlenose Pleco
Ancistrus sp.
A highly adaptable bottom-dweller that will not compete with surface-dwelling mollies and handles harder water reasonably well.
Great
Cardinal Tetra
Paracheirodon axelrodi
Require strictly soft, acidic water which is the polar opposite of the molly's physiological needs.
Not recommended
Tiger Barb
Puntigrus tetrazona
Can be overly boisterous and prone to nipping, which may stress mollies, particularly slower ornamental varieties.
Compatible with caution
Dwarf Gourami
Trichogaster lalius
Generally prefer softer water and a quieter environment; they may be stressed by the high activity levels of mollies.
Compatible with caution

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