Molly (Common / Sailfin) Care Guide

The molly is a hard-water, alkaline livebearer that grows bigger than most beginners expect and breeds faster than they plan for. Get the mineral content of the water right and almost everything else falls into place; get it wrong and the fish rock and sway their way to an early death.

Molly (Common / Sailfin) at a glance

The sourced figures the welfare engine uses to judge Molly (Common / Sailfin) — the parseable key facts.

Where it comes from

"Molly" is not one species but a small complex sold under one name: the common short-fin molly (Poecilia sphenops, Mexico south to Colombia and Venezuela), the American sailfin (P. latipinna, the Gulf and Atlantic coasts from North Carolina to Veracruz) and the giant Yucatan sailfin (P. velifera), all of which interbreed freely, so the "black molly" in the shop is usually a hybrid rather than a pure species. The wild biotope is warm, still-to-slow, heavily planted fresh water — streams, ponds, ditches and canals — that runs alkaline (pH around 7.5-8.2) and mineral-rich (dH roughly 11-30). Critically, the sailfins are also at home in coastal brackish estuaries, which is why mollies are euryhaline, moving between fresh and salt water, and why farms often raise them in brackish ponds. That origin explains the care: they want hard, alkaline, mineralised water and plenty of algae to graze, not the soft acidic water suited to tetras or discus.

Did you know?

• The name "molly" comes from a scrapped genus: the sailfins were once Mollienesia before being folded into Poecilia.
• It is a species complex, not one fish — most pet mollies are hybrids across P. sphenops, P. latipinna and P. velifera.
• Sailfin males raise a huge dorsal sail to court females and intimidate rivals; velifera carries the most fin rays (~18-19) of any molly.
• Mollies are euryhaline overachievers, ranging from fresh water to full marine salinity — which is exactly why the "do they need salt?" debate never dies.
• The genus includes the Amazon molly (Poecilia formosa), a famous all-female species that reproduces by gynogenesis.
• Both P. sphenops and P. latipinna are IUCN Least Concern; if anything mollies are too successful, going invasive where introduced.

Tank size — and why

Minimum tank depends on which molly you actually bought. A small group of common mollies wants 20 gallons (~75 L) as a floor; sailfins (latipinna and especially velifera) are better in 29-55 gallons and a long footprint such as roughly 90 cm. Three reasons drive the size: adult length (common mollies reach 8-12 cm but sailfins hit 12-15 cm and velifera females can touch 17 cm), the swimming room these active open-water grazers need, and the bioload — mollies are heavy eaters and high-waste, so they need volume plus strong filtration. Favour length and surface area over height; a sailfin male also needs horizontal room to fan his dorsal.

As a guide, a 20-gallon tank comfortably suits about 3–4 Molly (Common / Sailfin) as a single-species display, leaving room for tankmates.

More on numbers by tank size: How many Molly (Common / Sailfin) in a 20-gallon tank? · How many Molly (Common / Sailfin) in a 29-gallon tank? · How many Molly (Common / Sailfin) in a 40-gallon tank?

Water parameters in practice

Hardness is the parameter beginners get wrong and the one that matters most: mollies are an obligate hard-water, alkaline fish, not a soft-water community fish. Aim for moderately hard to hard, mineral-rich water with a pH of 7.5-8.5 and temperatures of 24-28 C (they tolerate down to ~18-20 C but dislike cool water). If your tap is soft, remineralise with crushed coral, a Wonder Shell or a mineral product like Seachem Equilibrium — do not just keep them in soft tap water and hope. Soft, acidic water is the classic molly killer; it triggers "the shimmies" (livebearer disease), a body-rocking osmoregulatory and organ breakdown. On the famous brackish question, present it as a range rather than a rule: modern consensus, including the breeder Goliad Farms, is that salt is NOT required — hard alkaline water is — and many commercial mollies, sailfins included, are raised without salt. Salt earns its place situationally: treating protozoan or fungal disease (~3-6 ppt), settling fish that were farm-raised in brackish water and struggle when dropped straight into soft fresh water, or a dedicated brackish setup for sensitive velifera. A minority, legacy camp argues black mollies and sailfins do best with salt; treat that as optional, not baseline.

Diet & feeding

In the wild mollies are strongly herbivorous — P. latipinna feeds almost exclusively on algae and plant matter, P. sphenops on green algae — so think of them as omnivores with a heavy plant bias. In the tank, give a quality flake or pellet plus a substantial vegetable and algae component: blanched spinach or veg, algae wafers, spirulina, and gel foods. Skimping on greenstuff stunts growth and, in sailfin males, blunts dorsal-fin development. Add frozen or live brine shrimp, daphnia and bloodworm in moderation. Feed once or twice a day, only what is cleared quickly; overfeeding fouls a high-waste tank fast. They graze biofilm and algae off glass, plants and decor all day, so let some algae grow — it is a feature for a molly, not a problem.

Gear & setup

Heated, hard-water, well-filtered and lidded is the brief. Run a heater for 24-28 C and strong filtration to cope with the high bioload; moderate flow is fine. Remineralise soft tap water with crushed coral, a Wonder Shell or Equilibrium to hold hardness and a steady alkaline pH. Plant densely, including floating plants — it matches the biotope, supports algae grazing and gives fry refuge from being eaten. Keep a tight lid: mollies are active and surface-oriented and will jump when startled, harassed or stressed by poor water.

Temperament & behaviour

Peaceful and gregarious in the right setup, but "peaceful" is conditional. Mollies turn nippy and bossy when crowded or underfed, mature fish may harass smaller tankmates, and sailfin males spar with one another during spawning. The biggest welfare lever is the sex ratio: males court and chase females relentlessly, so a male-heavy or cramped tank concentrates that harassment and stresses females, which can make them reabsorb a pregnancy or drop premature fry. More females, more plants and more length all dilute the aggression.

Group & social needs

Keep a group of at least 3, with 4-6 better for stability — but the sex ratio matters more than the headcount. Run 2-3 females per male (or keep males only) so chasing is spread out rather than focused on one female. Avoid pairing mollies with slow, long-finned, timid fish that boisterous mollies can out-compete or nip.

Compatible tank mates (preview)

A short, engine-cleared shortlist — the species TankStocking's welfare engine clears with Molly (Common / Sailfin) and that suit its size and temperament best. Tap any to load the pairing in the planner.

• Bamboo Shrimp (Wood/Fan Shrimp) — Peaceful temperament, similar adult size.
• Bleeding Heart Tetra — Peaceful temperament, similar adult size.
• Boesemani Rainbowfish — Peaceful temperament, similar adult size.

A note on the shrimp and snails here: Molly (Common / Sailfin) is peaceful and generally invertebrate-safe — but almost any fish will take very small shrimplets given the chance, so give shrimp dense cover (moss, leaf litter) if you want a colony to grow, rather than expecting every baby to survive.

See the full Molly (Common / Sailfin) tank mates guide →

Breeding & sexing

Sexing is easy: males carry a gonopodium (a pointed, rod-like modified anal fin), are smaller, slimmer and more colourful, and sailfin males fly the big dorsal sail; females have a fan-shaped anal fin and round out when gravid. Breeding is the opposite of difficult — mixed-sex mollies breed on their own, continuously. They are livebearers; a female stores sperm and can produce several broods from one mating, so even a fish bought "alone" can keep dropping fry. Gestation runs around 28 days (hobby range ~4-6 weeks; warmer water shortens it), with broods of roughly 20-100 fry that grow with the female's size and age. Fry are born free-swimming and eat immediately, but adults eat fry, so use dense floating cover or move the gravid female. Because stored sperm plus large broods plus continuous breeding fill a tank fast, plan ahead: males-only, rehoming, predators or culling.

Lifespan

Typically 3-5 years with good care, occasionally to about 6 in excellent conditions. The usual shorteners are self-inflicted: soft or acidic water (chronic organ stress and the shimmies), weak modern genetics from intensive colour breeding, high temperatures that speed metabolism, uncycled or overstocked tanks, and chronic female harassment in male-heavy groups. Balloon-morph mollies are congenitally deformed and die younger.

Common mistakes

• Keeping them in soft, acidic water instead of hard, alkaline, mineral-rich water — the classic molly killer that causes the shimmies and early death.
• Believing they "need salt": they need hardness and alkalinity, not necessarily salt, and salting a planted community can harm plants and salt-intolerant tankmates.
• Underestimating size — buying a "small" molly that becomes a 12-15 cm sailfin in a 10-gallon tank.
• Underestimating breeding: mixed sexes plus stored sperm plus 20-100 fry per brood means a population explosion and a bioload spike.
• Running an equal or male-heavy sex ratio so males harass females; use 2-3 females per male, or males only.
• Stocking a high-waste fish in an uncycled or under-filtered tank.
• Mixing molly species you do not want crossed — they interbreed freely, so keep one species for pure lines.

Signs of trouble

• Shimmying or swaying in place — the signature soft-water/osmoregulatory warning; fix hardness, pH and temperature first.
• Clamped fins, hiding or loss of colour.
• Hanging at the surface or sitting on the bottom, rapid gilling.
• Refusing food, sunken belly, stringy white faeces (internal parasites/wasting).
• White spots (ich), cottony patches (fungal/columnaris) or dusty gold film (velvet).

Is this fish right for you?

Don't buy a molly if your water is soft and acidic and you won't remineralise it — that single mismatch causes most molly deaths. Skip them if you want a no-babies tank but are buying mixed sexes, if you only have a small nano tank (especially for sailfins), or if you can't quarantine new arrivals. Avoid the balloon molly outright: the rounded, compressed body is a congenital spinal and organ deformity that impairs swimming, weakens the immune system and shortens life — it is an ethics problem, not a cute variety. Because mass-bred mollies can be genetically weak, buy body-normal fish from reputable local breeders or shops where you can.

Bringing one home

Quarantine new mollies before adding them — mass-bred imports often carry internal parasites or arrive stressed. If a fish was farm-raised in brackish water, acclimatise it gradually rather than dropping it straight into soft fresh water, and make sure the destination tank is already hard and alkaline.

Common questions

Do mollies need salt or brackish water?

No — what they require is hard, alkaline, mineral-rich water. Most modern sources, including commercial breeders, keep mollies without salt. Salt is optional and situational: disease treatment, a dedicated brackish setup, or rehabilitating brackish-farmed stock. A minority view favours salt for black mollies and sailfins, but it is not a baseline requirement.

How big do mollies get?

It depends on the type. Common mollies usually reach 8-12 cm; sailfin hybrids 10-12 cm; pure or large sailfins (especially velifera) 12-15 cm or more. The common quote that mollies "stay 7-8 cm" undersells the sailfin lines.

Why is my molly shaking or rocking in place?

That is "the shimmies" (livebearer disease) — usually a sign of soft, low-mineral, cold or low-pH water, not a contagious infection. Fix the hardness, pH and temperature first and add minerals.

How many mollies should I keep together?

At least 3-5, but the sex ratio matters more: keep 2-3 females per male, or keep males only, so females are not harassed.

How big a tank do mollies need?

A small group of common mollies needs about 20 gallons; sailfins are better in 29-55 gallons with a long footprint, because they are bigger, more active and high-waste.

Sources & confidence

Sources & confidence (9 species)

These back the Molly (Common / Sailfin) figures and the previewed tank mates above. Each figure is read from the TankStocking species database (v2026.06); below is the care reference behind it and how confident we are in that data. Confidence reflects the source quality, not whether any pairing is safe. Full source list and the welfare model are on the methodology page.

• Molly (Common / Sailfin) Poecilia sphenops / Poecilia latipinna — Aquarium Co-Op molly care guide / FishBase Poecilia latipinna high confidence
• Bamboo Shrimp (Wood/Fan Shrimp) Atyopsis moluccensis — Aquariadise (aquariadise.com/caresheet-bamboo-shrimp-atyopsis-moluccensis) high confidence
• Bleeding Heart Tetra Hyphessobrycon erythrostigma — Seriously Fish (Hyphessobrycon erythrostigma) high confidence
• Boesemani Rainbowfish Melanotaenia boesemani — Seriously Fish; Aquarium Co-Op Boesemani guide high confidence
• Bolivian Ram Mikrogeophagus altispinosus — Seriously Fish (seriouslyfish.com/species/mikrogeophagus-altispinosus) high confidence
• Brilliant Rasbora Rasbora einthovenii — Seriously Fish (seriouslyfish.com/species/rasbora-einthovenii) high confidence
• Bristlenose Pleco Ancistrus sp. — Aquarium Source / aqua-fish.net Ancistrus care guides high confidence
• Clown Pleco Panaqolus maccus — Fish Laboratory (fishlaboratory.com/fish/clown-pleco); AquariumStoreDepot high confidence
• Congo Tetra Phenacogrammus interruptus — Seriously Fish (seriouslyfish.com/species/phenacogrammus-interruptus) high confidence

Care-guide sources (9)

This guide synthesises the references below; where they disagree, the range and the disagreement are noted in the text above. The figures in the key-facts box are read from the TankStocking species database (v2026.06). Full welfare model on the methodology page.

• FishBase — Poecilia sphenops (common molly)
• FishBase — Poecilia latipinna (sailfin molly)
• Seriously Fish — Poecilia sphenops (Short-finned / Black Molly)
• Seriously Fish — Poecilia latipinna (Sailfin Molly)
• Seriously Fish — Poecilia velifera (Giant Sailfin Molly)
• Aquarium Co-Op — Care Guide for Mollies
• Goliad Farms — Mollies: To Salt or Not to Salt
• Aquarium Co-Op — Livebearer Disease
• Wikipedia — Sailfin molly / Poecilia velifera

More on Molly (Common / Sailfin)

Related guides on TankStocking — each scored by the same welfare engine as the planner.

This care guide is a sourced planning reference, not veterinary advice — individual fish, filtration and maintenance all matter. Cycle the tank, test your water, and observe your fish. How TankStocking works →