Molly

Looks how to spot one

Scaled livebearing fish

A sturdy, deep-bodied little fish with a small upturned mouth suited to grazing, a short dorsal fin (the feature that separates it from the sailfin molly) and a rounded to squared tail. Males are smaller and more vividly coloured, and their anal fin is reshaped into a rod-like mating organ called a gonopodium. Females grow roughly a centimetre longer and look fuller in the belly. In aquarium strains colour and fin shape are inherited separately, so breeders can combine them in many ways, including lyre-shaped tails.134

Personality what they are like

Calm and sociable toward other fish. Mollies potter around the tank nibbling at algae on every surface and generally leave peaceful tankmates alone. They do best with companions that enjoy the same hard, alkaline water, such as other livebearers, rainbowfish and hardy barbs.32

Daily activity how busy a day looks

Active mid-water and surface swimmers that spend much of the day browsing. Give them open swimming room as well as planted areas and surfaces where algae can grow.31

Grooming and care the hands-on bits

Water: about 18 to 28 C (21 to 28 C is a comfortable range), pH 7.0 to 8.5, moderately hard to hard water around 11 to 30 dGH. Mollies need hard, alkaline water rather than soft or acidic water, and they tolerate a little salt. Keep water clean with regular partial water changes, since they are sensitive to a build-up of waste compounds.234

Home where they are happiest

An aquarium with a base of about 90 x 30 cm (roughly 80 litres) suits a small group; FishBase gives 60 cm as a bare minimum length. Include dense planting, which also gives newborn fry somewhere to hide, and good filtration. In the wild they live in rivers, streams and coastal fresh or brackish water, with adults near the banks and young in very shallow margins.321

Food basics what goes in the bowl

Omnivore with a strong leaning toward plant food. Wild mollies eat algae, diatoms, detritus, small crustaceans, worms and insect larvae. In the aquarium offer a good flake or pellet with plenty of vegetable content, plus blanched spinach or vegetable flakes, and occasional small live or frozen foods.123

Company who they like around

Social fish that are best kept in a group. Keep more females than males (several females per male) so that no single female gets too much attention. Avoid mixing with sailfin mollies if you want to keep the species pure, since the two interbreed readily.31

The story where it all began

The French naturalist Achille Valenciennes described the fish in 1846. For a long time it was treated as one very widespread species with many local forms stretching from Mexico to Venezuela, and it collected more than a dozen other scientific names along the way, including Mollienisia sphenops and Poecilia thermalis. Today scientists split this group into a complex of about 13 related species, with P. sphenops itself restricted to Mexico, Guatemala and Honduras. The mollies as a group were first given the genus name Mollienesia by Lesueur in 1821, which now survives as a subgenus of Poecilia. In the aquarium world the all-black molly became a star. Its origin is debated: one account credits a New Orleans breeder, while others describe several breeders in Louisiana and Florida producing black fish independently in the 1920s and 1930s, and ichthyologist Eugene Balon traced the black molly to crosses between the short-finned and sailfin mollies. Fish sold as P. sphenops now rank among the most popular ornamental fish in the world.1456

Recognised by the official bits

IUCN Red List

Least Concern (assessed 22 March 2018)2

Taxonomy

Family Poeciliidae, genus Poecilia, subgenus Mollienesia (the mollies); now treated as one member of a P. sphenops species complex of around 13 species1

Aquarium trade

Fancy mollies are domestic strains, many derived from crosses of P. sphenops with the sailfin molly (P. latipinna) and, for high-finned forms, the Yucatan molly (P. velifera)4