First, you will need a two-gallon aquarium and a piece of black cardboard that can cover the back of the aquarium. Cut a hole about 1/2 inch in the center of the cardboard and place it on 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. Under these specialized conditions, the gravid spot of the females can be detected. Males will not have this spot. Segregate the males and
I found the above on the net. To me, sexing them is a pain in the butt!
You can wait till they get a little older than what it says above, when the males come to sexual maturity. They will develop a gonopodium(sp) it is a nifty third fin thing on their abdomen, that they use to deliver sperm to the female. Usually you can see the gravid spot on the females by then too.
Only thing is, if you wait that long to sex them, then some of the females may have already been impregnated.
Have fun! I don't envy you, LOL. :lol: