Common molly / short-fin molly (Poecilia sphenops) care guide

Quick facts

Adult size: 8 cm
Lifespan: can live up to 5 years; captive average 2-3 years
Tank zone: all
Temperament: peaceful
Difficulty: intermediate

Water parameters

Temperature: 24–28°C
pH: 7.5 to 8.5
Hardness: 15 to 30 dGH

Tank requirements

Minimum volume: 110 L
Minimum length: 75 cm
Flow: low
Lighting: moderate
Substrate: any
Open swimming room: needed

Feeding

Diet: omnivore, feeds primarily at the all.

Significant vegetable matter required: blanched zucchini, spinach, peas, spirulina flake. Without it they strip soft plants and graze biofilm aggressively. Grazes algae throughout the day.

Vegetable matter required (algae wafers, blanched zucchini, spinach).

Compatibility

Eats soft-leaved plants (cabomba, hygrophila, hornwort tips) when underfed on vegetables. Pair with hardy plants like anubias, java fern, vallisneria, and amazon swords
Tolerates and arguably thrives in slightly brackish water (1-1.5 tsp marine salt per gallon)
Hard alkaline water non-negotiable; will not thrive in soft acidic South American community setups
Keep 1 male per 3+ females; males harass females otherwise
Most ornamental varieties (black molly, dalmatian, balloon) are the same species or hybrids; care is identical
Safe with adult dwarf shrimp; may eat shrimplets due to larger mouth

Habitat

Native to Central American coastal waters from Mexico to Colombia, often in slightly brackish estuaries. Eurytopic; tolerates fresh through full brackish water and a wide temperature range. The 'balloon' variety is a deformed strain with a shortened spine and significantly reduced lifespan; the wild-type body shape is healthier.

Breeding

Livebearer. Females give birth to 20-60 free-swimming fry every 4-6 weeks with no special effort required. Males have a modified anal fin (gonopodium) used for internal fertilization. Females can store sperm and produce multiple broods from a single mating. Fry are large enough to eat crushed flake from birth. Population control is the real challenge; without predators or culling, a few mollies become hundreds within months. Males are persistent breeders and will harass females relentlessly; keep at least 2-3 females per male.

Common problems

Mollies are prone to a specific condition called "shimmies" (clamped fins, rocking motion, refusal to swim). This is usually caused by low mineral content or unstable water chemistry. Mollies do best in hard, alkaline water (pH 7.5-8.5, GH 15+). Some keepers add a small amount of aquarium salt (1-2 tablespoons per 20 L), which helps but isn't mandatory if the water is already hard. They're also susceptible to columnaris (cotton-mouth) and fungal infections.

Bioload

Bioload coefficient: 3.5 (8 cm livebearer with heavy plant matter consumption; pulled down from formula's 4.1 because mollies are mellow swimmers not active ones).

Bioload coefficients are calibrated against the neon tetra as the anchor (1.0). See the methodology page for the formula and how each value was derived.