Just got back from trading in 45 platys that grew from fry. Started with 3 platys a year ago, traded in 30 a few months back.
It helps to have a place for the fry to hide that the adults can't get to. I've never used a breeder trap, or taken the fry from the tank when they were young. If you make a good size pile of smaller rocks the fry can get in & out, while the adults can't fit. Works great.
You have to check in the rocks every day or two to make sure none of the adults have wedged themselves in there. They will try anything to get to the fry and eat them, and once in a while get stuck.
Tolak