Many of the common respiratory diseases are caused by viruses, and antibiotics will not help. The common ones are infectious bronchitis, infectious laryngo tracheitis or ILT, and some other lesser know ones. Viruses will have to run their course over severalmweeks. For mycoplasma or MG, the best ones are Tylan (tylosin,) tetracycline, doxycycline, and enrofloxacin. For coryza, sulfa antibiotics are usually prescribed by a vet. Many of the diseases listed can cause your entire flock to be carriers whether or not they get sick. You can read about those diseases and their symptoms here:
https://edis.ifas.ufl.edu/ps044
It can be hard to tell one disease from another without testing or a necropsy done after death, although listing all symptoms you are seeing may help.