sand filter with DE

I read some where on here about adding a cup of DE to your sand filter through the skimmer but now I can't find the thread I read it on. Can some one help me re find this information? Also, how do I know if the DE I find is pool safe, the pool store that sales it is an hour away and I'd rather just run down to lowes or other?

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I just re found the thread in How To...wish I new how to delete a post.

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Still don't know how to know if it is safe and not gardening DE
 
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