While your ducks could have one of many infections or viruses that they could have picked up from the environment, I think your ducks could have Avian Cholera, also caused fowl cholera.
This disease is associated with poor sanitation, mucous discharge, diarrhea and labored breathing. Lesions found in dead birds include hemorrhages on heart muscle and abdominal fat. The liver is enlarged, and small white marks can be seen on it. Good hygiene can prevent this disease from happening. Sulfadimethoxine-ormetoprim (0.02 - 0.04%) and Chlortetracycline (0.044%) given in feed are good treatments.
I would suggest you get one bird checked by a local vet to confirm this, and then continue treating them.
This disease is associated with poor sanitation, mucous discharge, diarrhea and labored breathing. Lesions found in dead birds include hemorrhages on heart muscle and abdominal fat. The liver is enlarged, and small white marks can be seen on it. Good hygiene can prevent this disease from happening. Sulfadimethoxine-ormetoprim (0.02 - 0.04%) and Chlortetracycline (0.044%) given in feed are good treatments.
I would suggest you get one bird checked by a local vet to confirm this, and then continue treating them.