I'm under the impression the birds have access to deep water during the day, correct?
Have you noticed any over-mating from your males?
Are the ducks acting lethargic, or showing any other upper-respiratory-related symtoms?
Respiratory diseases such as Mycoplasma Galliseptcum and infectious bronchitis are fairly uncommon in ducks compared to other fowl such as chickens. Which such diseases, I would also suspect other noticeable symtoms. In my experience, if the birds are acting ok, you're more than likely dealing with an environmental problem.
I would proceed with the treatment I suggested above, and try looking over their environment for any problems that may predispose them to develop such eye problems. That may include over-mating from the males, high ammonia, lack of deep water, mold, etc.