For roundworm infestation, Wazine works fine. It's also called piperazine. It only kills roundworms, though, nothing else. Valbazen kills all worms a chicken can get and starves them out over a several day period so they don't clog the system.
If they already have worms like that, Diatomaceous Earth is probably not going to do the job. It's good for killing soft bodied bugs in feed and nests and on birds themselves, but when wet, loses much of its cutting power so I wouldn't trust it to worm birds who already have visible worm loads.