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Family: Poeciliidae, Subfamily Poeciliinae

Common Names: Molly, Black Molly

Origin and Habitat: Mexico, Central America, to Venezuela and Colombia in northern South America; isolated populations on several Caribbean islands. Occurs in fresh and brackish water shallow streams, ponds and estuaries in coastal areas. Introduced into Montana, Nevada, California and possibly Florida [US Geological Survey].

Compatibility/Temperament: Peaceful, but should be kept with 2-3 females for each male if sexes are combined. Tankmates should be restricted to other livebearers, and peaceful fish such as some rainbowfish and those cyprinid and characin species that can manage in harder water.

Common Molly Diet

Naturally feeds on worms, crustaceans, insects and plant matter including algae. Requires vegetable and green matter in its diet, and should be fed a variety of good prepared foods, some of which are algae, kelp or spirulina-based. Algae should be encouraged as an important food source. Blanched spinach.


Males attain 3 inches, females close to 5 inches with some sources reporting up to 6 inches.

Minimum Tank Suggestion

24 inches but better in 36-inch tank.

Water parameters for Common Molly

Medium hard to hard (10-30 dGH), basic (pH 7.5 to 8.5), temperature 21-28C/70-82F. This fish cannot survive healthily in soft or acidic water. Many sources recommend brackish water, though in its habitat the fish generally prefers freshwater. The black strain will be healthier at the warmer end of the temperature range, and most sources recommend brackish water.


The common or short-fin molly is a hardy fish in the wild, but generations of tank inbreeding have resulted in fish that are weaker genetically and prone to disease. This fish should never be placed in a new "cycling" tank; it is highly susceptible to any level of ammonia and this will severely weaken its immune system, causing health problems and most likely a shortened lifespan.

The feral (wild) form has an olive-green back and bluish flanks spotted with orange, and the belly region is orange. The iris of the eye is blue; fins are brownish. The popular all-black molly was developed from this species; it is less hardy and requires warmer temperatures. There are now several varieties, such as the Gold, Green, Lyretail, and Dalmation, some of which are crosses between this species and P. latipinnis. The fish has an average lifespan of 4 years, except for the Black form which is about 3 years.

The "Balloon" variety is malformed and often develops internal problems with the swim bladder and digestive system leading to premature death; this practice should not be encouraged by purchasing this form of the molly.

The aquarium should be very well planted; the water must be hard and basic/alkaline, so plants like Vallisneria are ideal. Floating plants are advisable. Algae should not be removed from the sides and back of the tank because this is an important food source for this fish. This species swims in the mid-to-upper level in the aquarium, frequently near or at the surface.

Males are smaller than females, and the anal fin is modified into the gonopodium by which sperm is transferred into the female in "packets" called spermatophores. The female's eggs are fertilized and extra sperm is stored in the oviduct walls; the female can thus deliver up to three successive batches of fry without further insemination by a male. Gestation takes 3-4 weeks, after which the female can deliver a batch of fry, up to 120 each time, at intervals of 4-6 weeks. Breeding traps should never be used with the molly. All species within the genus will interbreed.

Fry are fairly large, but the parents will usually eat what they find; thick floating plants will help protect the fry. At birth, all fry appear to be females; by about 3 months of age, some of these will develop into males, but the majority (about 70%) remain female. Healthy development requires sufficient tank space from early on, with regular partial water changes.

The genus Poecilia contains several species of molly as well as the guppy and endler. A number of ichthyologists have recently proposed that the guppy does not share certain specific traits with the other species and should be separated. Poeser et al. (2005) suggested re-validating Acanthophacelus, originally erected as a genus by Eigenmann in 1907 and subsequently deemed a synonym for Poecilia, as a subgenus, and Schories et al. (2009) follows this but proposes that the entire genus Poecilia is in need of revision.

The subject species was described by A. Valenciennes in 1846. The genus name Poecilia was initially applied to the guppy and comes from the Greek poikilos meaning "with different colours." The species epithet is also Greek, meaning "with pointed head." The common name Molly comes from the scientific genus name Mollienesia that was erected in 1821 by C.A. Lesueur when he described the species Mollienesia latipinna; it honours one "Monsieur Mollien." Rosen & Bailey (1963) transferred the several "molly" species into Poecilia and the genus Mollienesia is no longer valid as distinct.


Poeser, Fred N., Michael Kempkes, and Isaac J. H. Isbrucker (2005), "Description of Poecilia (Acanthophacelus) wingei n. sp. from the Paria Peninsula, Venezuela, including notes on Acanthophacelus Eigenmann, 1907 and other subgenera of Poecilia Bloch and Schneider, 1801," Contributions to Zoology, pp. 97-115.

Rosen, D.E. and R.M. Bailey (1963), "The poeciliid fishes (Cyprinodontiformes), their structure, zoogeography, and systematics," Bulletin of the American Museum of Natural History, volume 126 (art. 1), pp. 1-176.

Schories, Susanne, Manfred K. Meyer and Manfred Schartl (2009), "Description of Poecilia (Acanthophacelus) obscura n. sp., (Teleostei: Poeciliidae), a new guppy species from western Trinidad, with remarks on P. wingei and the status of the 'Endler's guppy'," Zootaxa, No. 2226, pp. 35-50.

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