I don't know what it is but I would get some electrolytes you can purchase at a feed store and put in their water. Brewers Yeast that you buy in the health food store too might help them. 1 tablespoon per cup of food mix well. Do they have a coop that they go in at night? I would clean the coop out good.
How old are the ducks? Good luck with this I hope you don't lose anymore.