Guppies are prolific breeders, but your dreams of a tank full of generations of guppies can be quickly stomped out by guppies' cannibalistic tendencies. In a community tank, guppy parents as well as other species of fish pose a danger to fish fry.
Adult fish will eat both their own fry and the fry of other fish. This behavior is especially common in crowded tanks. Natural selection may also play a role. Fish fry who can avoid being eaten may be healthier, fitter fish who will improve the species. Parents may also eat unhealthy fish or cull the fry when there are too many. Cannibalism is a common occurrence in community tanks, and it is highly unlikely that you will be able to safely maintain guppy fry in a tank without at least a few fish succumbing to cannibalism.
Floating Breeder Tank
A floating breeder tank is a clear plastic tank that floats in the water. There is a divider between the top portion of the tank and the lower portion of the tank with tiny holes. When the mother gives birth, the babies fall to the bottom of the tank, secluding them from both the mother and other fish and ensuring that most of them aren't eaten. Some fry may still swim back up to the top of the tank, so this is not a completely safe method. It is, however, the easiest approach. When the female is pregnant, move her to the breeding tank a few days before giving birth. After she gives birth, remove the mother and fry from the breeder tank. Place the fry in a tank separate from the adults, and place them in the community tank when they are adult-sized.
If your guppy's pregnancy comes as a surprise and she gives birth suddenly, you must remove the fry from the tank to ensure their survival. Use a small fish net to catch the fry and transfer them to a separate aquarium. A few guppies may still be eaten unless you are present at the moment of birth to catch all of the fry. Fortunately, though, it usually takes a few days for all fish fry to be eaten by their parents or other members of the aquarium community.
The Natural Method
If you don't want to remove fish fry at all, there are a few steps you can take to increase the odds of survival for a few fish. First, ensure that your tank is not overcrowded. For breeding, you will need a minimum of 30 to 40 gallons to ensure success. Plant thick, movable plants -- real or fake -- at the bottom of the tank to provide the guppies with a hiding place. Feed the fish at regular intervals each day; this decreases the likelihood that they will chase and eat babies out of hunger. While this method can ensure the survival of a few fish fry, some fry may not be able to get sufficient food because they are busy hiding from adult fish.