When I used to breed guppies/platys/swords I custom made a "fry saver". I used what I think they call chicken fencing available at Home depot (at least they used to carry it). It was made of plastic and the holes were just big enough to keep the mommas out. I made a sorta stand which kept the momma in the top of tank but allowed the babies to fall to the bottom. I had a huge difference in survival rates this way.