Hi. We have had this issue a couple more times and dealt with it differently depending on the severity. If your duck's eye looks really bad, I would suggest separating it from the flock just so there's no chance of it spreading. A dog exercise pen is great for this or a rabbit cage will also work. Just make sure if possible, the cage or pen is near the others where the sick duck can still see them or else you msy have problems reintroducing him to the flock.
To treat this, use Veterycin eye drops, which you can pick up from your local feed store. Just be aware that the duck will shake once you put it in, so hold its bill while you apply it. I believe you use it for 7 days if I remember correctly.
Also, I don't know if you are already doing it, but you should also put in a bucket for the duck to wash his head in. Put some Apple cider vinegar in the bucket--1 tablespoon to one gallon. This will kill bacteria. Ducks need a way to clean their heads.
I know that I've been having issues figuring out how to do that with these freezing temps but you can either get a heated water bucket or use buckets in the day and dump them at night (also depending on how cold it gets where you are).
I don't know how you have handled this until this point since I see it's been a couple of days since you've posted, but if you decided not to separate this duck, you should still be okay as long as you start putting ACV in the water buckets and treating anyone else who has drainage from the eyes.
Hope this helps. Sorry I didn't see this until now.
Hope she continues to improve!
