I am not sure on the age of the ducklings as they came with two mother ducks. They are all different sizes so it is difficult to be sure on the ages. The skin did break which is why I removed the duckling as I felt its siblings were not leaving the wound alone. I am keeping the duckling in a shed and I take it into the main pen with its siblings four or five times a day but keep it away from the others at night. I will get some of the powder you suggest and I guess time will see if it improves.
Many thanks