Guppy

Poecilia reticulata is native to northeastern South America and the Caribbean but is now bred worldwide in countless colour and fin varieties. Males are small and gaudy with large ornamental tails; females are larger, plainer and rounder. As a livebearer, the guppy gives birth to free-swimming young rather than laying eggs.

Tank & water. Guppies stay small — males about 3 cm (1.2 in), females up to 6 cm (2.4 in) — and a group is comfortable in a 38–57 litre (10–15 US gallon) tank. They like slightly hard, alkaline water around pH 7.0–8.0 and temperatures of 22–28 °C (72–82 °F). Hard tap water often suits them without adjustment, part of why they are so beginner-friendly.

Temperament. Guppies are peaceful, active community fish. The main planning point is breeding: keep males and females together and they will produce broods every few weeks, quickly overrunning a small tank. Many keepers keep all-male groups for colour without the population boom, or accept that surplus fry may be eaten in a community setting.

Diet & care. They are omnivores and thrive on a varied diet of quality flake with occasional vegetable matter and small live or frozen foods; a little vegetable content suits their partly herbivorous gut. Colour and finnage have been selectively intensified over generations, and some heavily line-bred fancy strains are noticeably less hardy and more disease-prone than plain feeder-type guppies, so buy from a healthy source and quarantine new arrivals before adding them to an established tank. Females can store sperm and produce several successive broods from a single mating, so even a group bought as "all female" may deliver fry — a useful thing to expect rather than be surprised by.