I have no idea what Kent Nitrate Sponge is made of, but I can tell you that it brought my 55g tank's nitrate level down in 48 hours. Remove it at that point from the canister filter.
Now Kent's Phosphate Sponge is aluminum-based, and it affects leather corals badly.
You do need to determine what is causing the nitrates in your tank in the first place. Once you eliminate the sources, you can keep them down far more easily.