Observe the new fry for at least a week. Fry food is available at pet stores. It is powdery in nature, so avoid over-feeding as this has a tendency to clog the filter. If you observe any deformed guppies, remove them immediately. It is possible to begin determining the sexes and separating the fry after the initial week.
Experts have a procedure for sexing the fry that is easily reproduced by a home enthusiast.
First, you will need a two-gallon aquarium and a piece of black cardboard that can cover the back of the aquarium. Place a small lamp with a frosted 100-watt bulb directly behind the hole. Using the water from the aquarium containing the new fry, fill the two-gallon tank. Carefully remove two or three fry from the tank and put them in the sexing tank. Wait until early evening when the sun is dim, and using a magnifying glass, observe the guppy as it swims through the beam of light.