Welcome to BYC, sorry for your losses. I hate to say it, but it sounds like maybe it was duck plague:I have three ducks and I went out to check on them and they were out of water. One dead, one weak, and one fine. I automatically thought dehydration. The weak one wasn't convulsing and she was doing okay. I followed the instructions in this and then she got weaker, stopped drinking, and her nose stared bleeding. She died but I'm not so sure it was dehydration now. I'm really worried about my third duck because if it was illness, I dont want her getting it. Can anyone help?
https://www.northeastwildlife.org/disease/duck-plague
Where do you live?