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How would it be aspergillosis?
http://www.duckdvm.com/condition/aspergillosis
I think that if that is the case, maybe you should rethink your brooder set up. I'm not sure using a blanket for the substrate is recommended. It could promote too wet of an environment.
As well, those ducklings need to be able to stick their whole head in those containers. Their whole head means including their eyes and ears to rinse everything.
Their water should be changed out multiple times during the day. They need access to fresh clean water with no additives. If giving probiotics and/or electrolytes, that should be offered in a separate water container.
Sorry to hear about your losses. I hope you get this sorted out.
The brooder plate might be too low for them at this stage as well and they might not be staying warm enough when needed.