Guppies are non-aggressive and hardy fresh water fish, native to Central and South America. Guppies are attractive fish and add colour to an aquarium. These active little live bearers reach adulthood quickly and breed readily. They are popular with novice aquarists because of their many positive attributes, but once guppies begin to breed, an aquarium can soon become over populated. Each aquarium can house only a select number of fish and it is therefore necessary to stop guppies from breeding on an ongoing basis.
Separate all the male fish from the female fish, by placing fish of one sex in a separate aquarium or by placing a divider into the original tank. Male guppies have a modified anal or ventral fin, which is termed a gonopodium. This fin, through which sperm is deposited into the body of the female, is long and pointed. The female has a rounded anal fin.
Place a plastic partition in the aquarium, to divide the tank in half. Use a perforated partition, so that water can pass from one side of the aquarium to the other. In this way, the temperature remains constant on both sides of the tank and the entire aquarium is correctly filtered.
Catch and remove fish of one sex to a separate aquarium, or to the other side of the partition.
Observe the females carefully. Female fish can store semen from males for half a year and can therefore give birth six months after being separated from the males.
Look out for a dark area around their anal or ventral fin of the female fish. Guppy females give birth to live young and exhibit this darkened pigmentation prior to birthing. Remove such pregnant female guppies from your display tank.
Remove all plants and other decor from an aquarium in which a female guppy is about to give birth. Guppies sometimes eat their own young, as do other fish in the same aquarium. By removing plants in particular, you are preventing the day-old guppy fry from hiding and many will be eaten. This is a natural approach to population control.