In the picture posted of your rooster that died last week, his one eye that is visible appears grey. Is it possible that he died of Mareks disease? It is hard to know which respiratory disease you are seeing, but all respiratory diseases make carriers of survivors, as well as the rest of your chickens. To get rid of the disease, you will have to wait until all birds have died, and then disinfect your facilities, and wait several weeks to get new chicks. But in your area, it may be difficult to find chicks who are healthy and free of disease.
MG, a common disease is passed through hatching eggs, and if the chicks are from an owner who has had MG problems, then all of their chicks sill be carriers. Birds with MG will not drink or eat well. I would just treat your remaining birds with the antibiotic, and try to treat crop croblems as they happen. Sick birds develop crop problems or slow crops. You should close your flock to any new birds, because each new addition will probably get sick as well. I am sorry that you are seeing so many problems with your chickens.
