I think you need to cull the entire flock. You will not be able to bring any birds in or let any leave if you do have a contagious respiratory disease. I would contact the local agriculture extension to have a few birds tested (you will have to pay for it) to be sure that is what you have. If you have a contagious respiratory disease and it certainly sounds like you do you must cull every bird (with or without symptoms as they ave shared the same air space), burn the bodies (the state may do this for you for a fee), clean out the coop with a bleach and water solution then lime it, after 3 or 4 months clean it again and then you can begin to rebuild the flock. I know this is hard to hear but this is the reality of what you may be dealing with. Like Cynthia said Tylan 50 and other antibiotics will only cover up the disease per say but the birds will ALWAYS be sick and will infect any bird brought on to your property. If you intend on breeding or selling birds then those that leave will most likely infect the flock of the person that bought from you. I like Cynthia and others do not medicate my birds with antibiotics. IMO it makes for a very weak flock that will be easily susceptible to disease.
This post is my personal opinion and I do not mean any disrespect nor to upset you.
Good Luck
This post is my personal opinion and I do not mean any disrespect nor to upset you.
Good Luck