Molly

About the species

A molly (Poecilia sphenops) kept in water that is too soft or acidic weakens quickly – fish start swimming at an angle, develop fungal patches on the fins, or show clamped fins. The cause is almost never disease but the wrong water hardness: this livebearer originates from mineral-rich waters between Mexico and Colombia, and in the aquarium it needs the same – hard, alkaline water rich in calcium and magnesium, not the soft water many people default to for a tropical tank.

Almost every molly sold today is a bred form (black, gold, sailfin, lyretail, balloon ...); true wild-type fish are rarely seen in shops. Years of breeding from a narrow gene pool have also left many commercial lines genetically weaker and more disease-prone than the wild form.

Hard, alkaline water is a requirement, not a suggestion

Recommended pH sits between 7.0 and 8.5, with general hardness (GH) between 15 and 30 °dH – considerably harder than what most tropical aquarium fish need, and harder than related swordtails or platies. Carbonate hardness (KH) should be high enough to buffer against pH drops, roughly 10–15 °dH. Salt is not required – it's the hard minerals, not salinity, that matter, though mollies also tolerate slightly brackish water well and are a popular choice for cycling new marine tanks.

Space and equipment

For a small group (ideally one male with two to three females), a footprint of at least 90 × 30 cm is sensible, holding roughly 80 litres. Filtration should turn over the volume four to five times an hour, and temperature should sit between 22 and 28 °C. Given adult size (males reaching around 8 cm) and active swimming, small, shallow tanks are not suitable long-term.

Why 'beginner fish' only applies with a caveat

Shops often recommend mollies to complete beginners, but that only holds conditionally – the fish is easy to feed and breeds readily, but is demanding about water chemistry. A keeper who fills the tank with soft tap water without checking hardness quickly runs into recurring health problems that are easy to misattribute to food or quarantine rather than water chemistry.

Suitable tankmates

Mollies are peaceful, but their need for hard water means they aren't a universal community fish – they shouldn't be mixed with tetras or other species that need soft, acidic water. They pair better with other Poecilia species, hardness-tolerant East African cichlids kept separately, or hard-water-tolerant barbs and tetras.

A diet with real plant content

In the wild they graze on small invertebrates and detritus; in the aquarium they need a regular vegetable component – blanched spinach or quality algae-based flake. Without enough plant matter, mollies can develop digestive problems.

Breeding without any encouragement needed

Mollies are livebearers – gestation runs around 8 weeks, and a single brood can number up to 120 fry. Adults will eat fry in the main tank, so keepers often move a gravid female to a separate tank before she gives birth. Because males pursue females almost constantly, a ratio of at least two to three females per male is recommended.

The balloon form and other varieties

Among the bred forms is the so-called 'balloon' molly, with a rounded, shortened body. This deformity often causes swim bladder and digestive problems and is linked to a shorter lifespan – it is a cosmetically bred form, not a health-neutral variant of the species.

Pros & cons