I have a flock of 18 3 month old chickens. They were hatchery bought and vaccinated. They are in a large coop and house. We are on property that hasn't had chickens on it in years. Their house and coop are new. They are eating started and get grit and oyster shells. They look very healthy Infact they look much older. The problem is most seem to keep diarrhea. What can I do? I have already wormed them.