Guppies
Livebearers: the fry arrive swimming rather than as eggs, about a month apart. The colour is on the males, and a mixed tank fills itself quickly.
Overview
Guppies are small livebearers from northern South America and the Caribbean, males to about 5 cm and females to 6 cm, and they are the fish most likely to turn one purchase into a population. A female stores sperm and can produce young every four weeks, delivering 20 to 40 live fry at a time after a gestation of four to six weeks.
That is worth planning for before buying rather than after. A mixed sex tank does not stay the size it started at.
Temperament
Constantly moving, peaceful, and entirely uninterested in anything except food and each other. Males display at females continuously, which is why the usual ratio is two or three females to each male: a single female with several males gets no rest.
They are also unbothered by an audience, so a guppy tank is active whether or not anyone is in the room. Nothing about them is territorial and they mix with almost any fish that will not eat them.
Care and grooming
Zooplankton, small insects and detritus in the wild, so a good flake and micro pellets, with occasional frozen food.
FishBase gives a temperature range of 18 to 28 degrees C, which is 64 to 82 F, with 23 to 24 C stated as the optimum, and a pH of 7.0 to 8.0 in moderately hard water. Guppies are frequently described as hardy and frequently sold in poor condition, and the two facts together explain most early losses in a new tank.
Living and exercise
A 60 cm tank, roughly 10 gallons, planted around the edges with open water through the middle. Fine leaved plants or floating cover give fry somewhere to survive, and a bare tank means the adults eat most of them.
If the population is meant to stay stable, a single sex group is the straightforward answer. Otherwise expect to rehome regularly, and check that whoever takes them wants them, because guppies released into local water have established themselves in warm climates around the world.
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