I've been adding a sponge to my box filters for many years now.I separate my fry from their parents right away and place them in very small tanks,2 and 1/2 to 5 gallon.I feed them a lot of very fine live foods ,newly hatched brine shrimp,Microworms,and Paramecium.I don't want to feed these food to breeding size fish.This is the perfect filter for these small tanks.On top of the sponge you can put crushed coral or activated carbon and on top of that filter floss.
Good idea Copper. I go one step further to make good use of that box. I have been loading mine with biomedia topped off with floss. The biomedia beats carbon and such in most situations and the floss keeps fine particles and fry out. It is not a sponge filter but serves quite well with fry.