Guppies are part of the group of tropical fish known as livebearers, meaning that they give birth to live, swimming babies instead of laying eggs. If you watch your guppies closely, it may seem that the females are always pregnant, which is actually pretty accurate.
Any time male and female guppies are together you can figure that the female is going to get pregnant. Rather than having to create just the right conditions to encourage them to breed, as you have to do with some types of fish, the trick with guppies is sometimes to get them not to breed, and that means separating them. If you keep them together, a normal female will get pregnant. If you have a group of guppies in your aquarium, you won’t be able to tell who the father is, because all the males will pursue all the females.
To be able to control when your guppies breed, separate them when they are young. Identify the females by looking for the dark spot, called the gravid spot, on the underside, just where the tail starts. Guppies can get pregnant as early as three months of age, so it’s best to separate them well before that time. Watch the male for the development of his gonopodium, a special fin set under his belly. When it begins to look long and pointed, he can impregnate females, and is a sure sign that your fish should be separated.
If you wonder how guppies can be pregnant all of the time, blame it on superfetation. This process allows female guppies to store unused sperm and then use it later, when it’s needed. Female guppies can hold onto sperm for anywhere from four to eight months, and then use the stored sperm to get pregnant again as soon as they’ve given birth. If she’s exposed to a new male, the female may end up being impregnated by him instead of the stored sperm, but it’s usually difficult, if not impossible, to tell for sure who fathered the latest batch of babies.
The guppy’s usual cycle of pregnancy and birth takes about four weeks. She doesn’t have a lot of down time between giving birth and starting again, so typically you can expect that a normal, healthy female is going to deliver a new batch of fry, the name for baby fish, a little more often than once a month. If you want to keep a female separated until you have the right male for her, keep in mind that if she is separated from males for too long, she may begin to have trouble getting pregnant in the future.