I'm with you, there are some things that just aren't treatable and euthanasia is the clear choice when an animal is suffering.As hard as it is, you did the right thing. I don't own ducks but do have chickens. After letting one with Ascites die of it, I won't do that again. My chickens aren't my pets, but I won't let a pet suffer either. Sometimes the kindest thing for them is the hardest for us.
