Hard to tell what exactly is going on. Could provide a little bit more information about the ducks?
Do you know what they were being fed, what they were being housed on/in? Are they pooping at all, if yes, could you post a picture? Are they eating/drinking much? Moving around?
Try to grab one, and do a nice thorough physical examination, checking around their vents, their abdominal area, mouth, feel their legs for swelling, make sure each joint bends, etc. Check their footpads, for signs of bumblefoot.
For now, since they're feeling under the weather, keep them in their own little pen area, with feed and water provided. If they're not drinking much, try dripping a few drops of Gatorade, or sugar water along their bill. Once they are feeling better, you can integrate them, but If their already feeling under the weather, integrating them may cause more stress.