The others are fine. I just dont see how he could have eaten something bad because tye ducks are in a winter dog run and dont have access to lots of land. He did have crusty eyes and maybe had a little blindness. Do you think it is a disease? A few weeks ago i had a duck put down, but it was acting normal and eating, just lame beacause of birth defects.
There are so many things that can happen. He could have had a systemic infection and hidden it from you till it was far advanced. He could have had a mild virus, but got run down, and had a stroke.
Often it is not. Just keep a close eye on them - you may want to change out bedding, scrub food and water pans, that kind of thing just to be on the safe side.
You may also consider sending a fecal sample to a vet for testing - it usually doesn't cost very much just for the sample to be tested.