Do you have a sump or wet/dry? Or is the LR
your primary filtration?
If you have sufficient LR
(how much?), you would be better off without the sponge unless you clean it on a regular weekly basis. The canister is still a great place for "occsional" products like resins and such or for people like myself that use carbon (chemi-pure) on a regular basis. If running empty, it can also make a great extra PH
Could you give a little more detail on the tank set up?