A sturdy, adaptable livebearer available in many colors and fin types, well known for tolerating a range of water conditions including light brackish setups.
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Educational only. KinStation content is reviewed by licensed veterinarians but cannot replace an in-person exam. Always consult a licensed veterinarian or board-certified specialist for diagnosis, treatment, or any decision affecting your pet's health.
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Around 6-10 cm total length depending on strain and sex
Lifespan
3–5 years
Social needs
group
Native region
Atlantic slope of Mexico and Central America (south to Honduras and Guatemala); introduced widely elsewhere
Origin
New World
Climate
🌴 Tropical
Water type
💧 Freshwater
Family
Poeciliidae
Genus
Poecilia
Part of the Livebearers
Egg-free breeders that give birth to free-swimming fry — guppies, mollies, platies, swordtails and their dwarf relatives. Hardy, prolific, and beginner-friendly favorites of the freshwater hobby.
From the minimum an animal needs to be kept humanely, up to the ideal setup. Bigger is almost always better — minimums are floors, not targets.
Photo coming soon
Minimum
Hard-water community
20 gal long / 75 L
Mollies are robust livebearers but get big (8–12 cm) and produce a lot of waste. Hard, alkaline water (pH 7.5–8.5, dH 10–25, 24–28 °C); some keepers add 1–2 tsp salt/gal for sailfins.
Photo coming soon
Recommended
Group / brackish-leaning
29–40 gal / 110–150 L
Group of 1 male to 2–3 females avoids harassment. Strong filtration, robust plants (vallisneria, anubias), and varied diet (veggies + protein). Fry are eaten by adults.
Photo coming soon
Ideal
Brackish biotope
55 gal+ / 200 L+ brackish
Long brackish (sg 1.005–1.010) biotope with rocks, vallis, and a sand bottom. Healthiest sailfin mollies — full dorsal display, vivid colour, fewer disease issues.
Life & growth stages
How this animal changes through its life — each stage often has its own care, diet and space needs.
Photo coming soon
Egg
Fish eggs are small, translucent spheres, often laid in clutches on plants, substrate, or in a nest — or carried/brooded by a parent in livebearing and mouth-brooding species. A dark eye spot and the curled embryo become visible inside as development progresses.
Photo coming soon
Fry
Newly hatched fry are tiny and semi-transparent, frequently still carrying a yolk sac that fuels them before they feed freely. They lack full fin structure and adult coloration, staying near cover until they can swim and forage on their own.
Photo coming soon
Juvenile
Juveniles look like miniature adults but with developing fins and muted or different markings; many species shift pattern and color as they mature. Growth is rapid at this stage given clean water and steady feeding.
Adult
Adults show the species' full size, finnage, and mature coloration, and are sexually mature. Many fish develop sex-specific differences in size, color, or fin shape, which can intensify during breeding.
Color & pattern variants
Natural variants occur in the wild; selectively bred (man-made) variants were developed in captivity.
Mollies are active and grow larger than guppies, so a group needs a minimum of 75-115 liters (20-30 gallons), with sailfin types needing more. Provide a fine substrate, robust plants or algae-covered surfaces for grazing, and open mid-water swimming space. Good filtration and surface agitation are important because mollies have a relatively high bioload.
They prefer warm, hard, alkaline water: 24-28 C (75-82 F), pH 7.5-8.5, and high hardness (GH 15-30). Mollies tolerate and often appreciate a little salt and even acclimated brackish conditions, but this is optional in a hard freshwater planted tank. Stable parameters and weekly water changes prevent most issues.
Substrate
Fine sand or smooth gravel works well, and a dark substrate shows off their coloration while letting them graze algae and biofilm off surfaces. Robust plants and some hardscape provide cover for fry and resting spots.
Equipment & setup
Provide a heater set to 75-82F and a filter that handles their heavy bioload; mollies appreciate good surface agitation and well-oxygenated, hard, alkaline water. Moderate to bright lighting encourages the algae and biofilm they love to nibble.
Diet
Mollies are omnivores with a strong herbivorous tendency. Offer a spirulina or vegetable-based flake or pellet as a staple, plus algae, blanched vegetables (zucchini, spinach), and regular grazing surfaces. Supplement with frozen or live daphnia, brine shrimp, and bloodworms for protein.
Adequate plant matter and fiber are essential to prevent constipation and bloating, which mollies are prone to on an all-protein diet. Feed modest amounts once or twice daily.
Behavior & temperament
Mollies are generally peaceful, social shoaling fish that do best in groups, ideally with more females than males to spread out male attention. Larger males can be mildly territorial or pushy, so provide space and visual breaks with plants and decor.
They are confident, busy swimmers that spend much of the day grazing surfaces. Keep them with similarly sized peaceful community fish; very small or long-finned tankmates may be harassed. Enrichment comes from algae to graze, planted cover, and gentle water movement.
Health
The classic molly ailment is 'shimmies' or molly disease, where the fish wobbles in place without moving forward, usually caused by poor water quality, low hardness, or temperature stress rather than a single pathogen. Ich, fin rot, and fungal infections also occur with stress or injury. Black mollies in particular are sensitive to soft, acidic, or cold water.
Prevention centers on warm, hard, alkaline, stable water, a fiber-rich diet, and avoiding overcrowding. Quarantine new fish. This is general care information for vet review and is not a substitute for diagnosis by an aquatic veterinarian.
Tips, DIY & hacks
Mollies tolerate and even benefit from slightly hard, mineral-rich water; some keepers add a little aquarium salt, though it isn't required in hard water. As prolific livebearers, add dense plants or a breeding box if you want to save fry, and avoid overstocking to manage their waste.
