A quick inexpensive way to determine if your chickens are coming down with coccidiosis is to take a sample of poop from the chicken with the most symptoms to your vet and request a fecal float test for parasites.Did the corrid help? I have a hen with similar symptoms.
This test can be done in an hour and is quite affordable and conclusive. You will know if your chickens have a high number of coccidia in their intestines and/or worms. Then you will know what to treat them for removing all guess work.