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Determining the sex of a guppy isn’t too hard, since males and females have a few distinct differences, but you may not be able to tell them apart until they are old enough to display their distinct gender characteristics. Once you can tell males from females, it’s a good idea to separate them by sex, since guppies are prolific and will happily fill your aquarium with small copies of themselves.
Observe the guppies coloring. Males typically have more color and brighter color. The males also have long, flowing tails, often as long as or longer than their bodies. Since fantail guppy females can be fancy, too, the males' brighter body colors and long, flowing tails can be indicators of gender, but they aren’t always definitive.
Look at the overall shape of the fish's bodies. Males are typically long and slender. Females are bigger and have heavy, rounded bodies compared with the males'. Look closely at their undersides. A female has a dark spot, known as the gravid spot, by her triangular anal fin. The spot gets darker the closer she is to giving birth. The male’s anal fin, called a gonopodium, long and thin, is specialized for delivering sperm to the female.
- International Fancy Guppy Association: Beginner’s Guide
- Water Gardens Online: Tropical Fish in the Water Garden
- Guppies, Mollies, and Platys: Everything About Purchase, Care, Nutrition, and Behavior; Harro Hieronimus
- Jochen Sands/Digital Vision/Getty Images