Instructions
Calculate the last time there was a birth in the aquarium. Guppies become pregnant every 30 days and have a gestation of 28 days. Mollies can gestate more than 100 days. If there was a recent birth and a subsequent birth occurs about one or two months later, the mother is most likely a guppy.
Locate the guppy and molly in the tank. Look at their anal areas and find their gravid spots. The gravid spot becomes darker as the pregnancy wears on and stays dark for a while after a pregnancy. A dark spot by the anus of either the molly or guppy means that fish recently gave birth.
Check to see if one of the fish is pregnant. Look at their stomachs and if one is distended or you can see movement, then that fish is pregnant. The pregnant fish also eats more than usual. Guppies are breeding machines and mollies get pregnant easily as well. If one of the fish is pregnant, then the other recently gave birth.
Watch how the guppy feeds after you first see the babies. The mother has a hormone that suppresses eating for a short time, about 12 hours, after giving birth to keep her from eating her young. If the guppy doesn't eat, then she recently gave birth.
How Do I Know If it Is My Guppy or Molly That Gave Birth?
Guppies and mollies are two common aquarium fish that are prone to frequent pregnancies. In fact, guppies get pregnant about every 30 days. In tanks stocked with both fish, it is even possible for the two species to mate and create a hybrid. With such high birth rates, difficulty may arise figuring out which species gave birth. The babies are too small to discern their species, so it's up to examining the adult fish.