You might read http://pool9.net/de-test/

The page describes adding DE to your sand filter, to verify that it is working properly. BUT, DE will also cause a sand filter to capture smaller particles than it otherwise would do, so using DE might reduce the time it takes to clear the water. However, it will result in more frequent backwashing, and you'll need to replace the DE each time. For most residential sand filters, 1 - 2 cups of DE is enough.