The first thing we did with our Platys was switch over to flat glass marbles for the substrate (gives them places to hide from the big fish). In one corner, I put a lot of plants close together to give them a quiet corner to drop fry when the time came.
After a female dropped fry, I would take the adults out, then the decorations and marbles (carefully).. putting the fish in 1 covered container and the other stuff in another container with enough tank water to cover it all. Then I could find the fry and move them all to a cup that I floated in another tank that we had ready for the fry.
The fry tank has a
bare bottom with a few plants and caves. The filter would be either a sponge filter or a regular filter with fish net netting tightly wrapped around the intake and held in place with a rubber band.