mature/established sponge filters have lots of little yummies on them that fish, especially fry, love to pick at. inverts like shrimp are fond of feeding off a sponge filter too.
++1 to sponge filters, with several clear advantages, They don't suck up the fry, they help to aerate the tank and huge surface area for bacteria.... and like said earlier, a great feeding ground.
Cheers!!!
sheldon