Actually these sponges were not getting really dirty. My water flows over my overflow, through sponges wrapped around the return pipe, into my fuge, through some rolled up mesh, through the fuge with tons of chaeto in it, then over a divider where it then ran through the sponges to be skimmed and then returned to the display. The really nasty sponges are the ones in the overflows which I clean every week. I don't always use saltwater to clean these and am now wondering if rinsing with tap water - granted from my RO tap is perhaps adding to my nitrate. It is still low today BTW and am still running carbon. Input welcome!
