I find Baycox (tolrazuril), which you have to get from the vets, way more effective than Coxoid (amprolium) for actually treating coccidiosis. Particularly if you're going to go to the bother of actual dosing them (which I think is a good idea).
Also amprolium is certainly a problem for ducklings when it's in chick crumbs, don't know if it is for adult ducks too.