DE stands for diatomaceous earth, and it is the stuff that is used in DE pool filters. You can get it from the pool store, or from Lowe's or Home Depot, and I have even seen it sold in smaller bags in Walmart's pool section in previous summers.

You should not see anything shooting back into the pool from the returns except water. If you do, then you have a filter issue that needs to be addressed. If you don't, then that's a good thing! You can add a handful through the skimmer, give it about 30 minutes and check your pressure--you're looking for no more than a 1 psi rise in pressure. The DE will help coat the sand, and will help catch the finer stuff. It will wash out when you backwash, though, so you'll have to add more after a backwash.

If the pressure builds while it's running, and goes down after a backwash, then you're doing what you need to do--keep filtering and backwashing as the pressure indicates, and the pool will clear, you just gotta be patient!

Janet