Molly

The common molly, Poecilia sphenops, is a hardy livebearer from the fresh and brackish waters of Mexico and northern Central America, reaching into Guatemala and Honduras. Wild fish are a plain silver-grey, but the trade is full of crossbred strains: the popular black molly, plus orange, dalmatian, and lyretail forms produced by crossing P. sphenops with its sailfin relatives. Adults reach about 7 to 8 centimetres, with females growing larger and rounder than the more colourful males.

Mollies are peaceful, active omnivores that graze constantly on algae and biofilm and also take flake, pellet, and vegetable foods along with the occasional frozen treat. They are social and best kept in groups, ideally with more females than males, because males court persistently. They mix well in a community of similarly sized peaceful fish. Since they prefer hard, alkaline water, they are a poor match for soft-water species, and some keepers add a little aquarium salt, though that is not strictly needed in suitably hard water.

Provide a 60 litre or larger tank of hard, alkaline water around 24 to 28 degrees Celsius at a pH near 7.5 to 8.2, with open swimming space and some plants. The main thing to expect is babies: mollies are prolific livebearers that give birth to broods of twenty to over a hundred fully formed young after about a four-week gestation, and a female can store sperm to produce several broods from one mating. In a community tank most fry get eaten, but in a planted or species tank populations climb fast, so plan for steady reproduction.