I get my DE at a health food store. I always look for the package to say "Food Grade."
I mix a heaping Tablespoon full in a gallon of feed and give that to them 3 days in a row, every month. I also mix a TBL. in a glass of water and drink it myself.
It's not only good for parasites, it has Silica in it. That's something many people are deficient in. Whenever I get a flare-up of Bursitis DE takes care of it in one day.
I also dust the chicken coop floor and nest boxes before adding new straw.
Just be careful that it never gets in your eyes or your birds eyes.
I even mix a little in my dogs food. Never have any of my animals had worms.
It's wonderful stuff!