Alright.. DE is absolutely fine but if you are looking for proven results then you would want to deworm with something like ivermectin or panacur. You can use a cattle or sheep drench with ivermectin and give orally. I would suggest using ivermectin orally or topically in the future to get rid of mites and worms in the future. Ducks only take a load of mites when they are unwell. It's a good warning sign. I wouldn't take a duck to a vet to be wormed. In the future, I would look for the many signs of worms, offer squash seeds seasonally that act as a natural wormed and improve their environment. Ivermectin works, panacur is easier to dose.