I have used the cellulose fiber material and if you haven't, DON'T. Sorry for shouting.

It did nothing to help the filtration and it formed big clumps in my filter and doesn't backwash out. Many cellulose fiber clumps are still there years later. I remove the top of the filter when winterizing and remove chunks of that junk every season - it looks like a newspaper left in the driveway in a snowstorm and driven over for a week.

Leslie's told me that DE was illegal and that the cellulose fiber was the only alternative - This is not true.