Do you have Tylan to put into the water? I would run a course of antibiotics through them all for 10 days. Fresh water and meds every day - yes every day. See how they are after that. If they are improved but not 100% , then wait a week and do it all again. If not better after second round, then may need to get a vet in to run some tests. Respiratory disease is pretty common in fowl but some are more serious than others. The saying is ...when you hear hoof beats think horses not zebras. So treat for the horses and if they don't improve then start thinking about the zebras.