My MaxiJets came with two different types of plastic filter type things that go over the intake...if one of those are in place I wouldn't think a sponge would be necessary.
Hope this helps!
Robbie
If you choose to use the sponge though make sure you stay on top of cleaning it as it can turn into a nitrate factory with waste and food particle build up. But like stated above it isn't needed but something should be placed between the livestock and the spinning hardware.