Not to play devils advocate, but I have a vertical slit breeder box and a grate style breeder box and my guppy fry always drop into the separate compartment and then decide to go exploring, back into the upper compartment with their mommy in it. Cost me at least 1 fry the last time.
T'was feeding time and I noticed a fry in the upper compartment with its mommy. Before I could go and get the multi functional suction/cleaning tool (aka turkey baster), she decided she needed a snack, right then, right there.