I have used sulfamethazine in their water as suggested by my local feed store where I bought some of my chicks that are now 6 months old, they act lethargic for a few days and weak, then they die. They die one chicken at a time about every 10 days. So far its been all my 6 month olds, not my older chickens, but last 2 days one of my older chickens is acting strange, lethargic and not interested very much in food, sort of uncoordinated. Another piece of advice I had from a feed store chicken person was to do another round of antibiotic along with wormer that goes in their food. Does anybody know what would kill one chicken at a time about every 10 days?