Is this sponge filter working?

I have a power head where the water goes in all around the bottom, unlike a traditional one. It is right inside a piece of filtering sponge in a little crevice I made for it. I have it sitting in the corner of my tank with rocks all around the bottom so no water gets through without going through the sponge. So it has to be working because water comes out of the other end, but when a little piece of old food or something floats by it doesnt get sucked into the filter, but big pieces of stuff get sucked onto it.