Worming chickens

I've got just a backyard flock of chickens, and have recently added two more RI Red hens to it. These hens are exactly the same age as the other birds, but they're sort of scrawny compared to mine. Not sure why; I guess my own chickens get the royal treatment, food and range-wise. I've decided to worm the lot, and wonder if anyone's had luck with any of the diatomaceous earth products (like PoultryGuard) out there.