In any case of respiratory illness, it's important to know if you're dealing with a viral, bacterial, fungal or parasitic disease. To make a diagnosis, an avian veterinarian (please see here: www.aav.org) will perform several tests including bacterial cultures of the airways, blood tests, and necropsies of dead birds is they're available. Microscopic evaluation of affected tissues is helpful and can be performed at a diagnostic laboratory. A fecal test for parasites also should be done. Attempts to isolate virus may be required.
But what if such a vet isn't available? I might conjecture that your chickens are suffering from infectious bronchitis which is caused by a corona virus. Infected birds usually cough because of excessive mucus in their trachea. This disease spreads rapidly through the entire flock but in uncomplicated cases the flock recovers quickly. There is no effective treatment. Prevention is by vaccination.
Other respiratory agents are less likely. The bacterial fowl cholera (Pasteurella multocida) usually manifests as unexpected deaths, swollen sinuses and wattles. The bacterial chicken coryza (Haemophilus paragallinarum) produces foamy, watery eyes, discharge from the nostrils, and sometimes swollen sinuses. Unlike fowl cholera, deaths rarely occur. Avian mycoplasmosis (Mycoplasma gallisepticum) produces respiratory illnessess indistinguishable in most cases from chicken coryza.
If you believe that these infections other than infectious bronchitis are possible, presumptive therapy with tylosin (Tylan-50) dosed at 20-30 mg/lb intramuscularly or orally for 5-7 days or tetracycline (Duramycin-10) dosed in the drinking water as per the label's instructions would be reasonable. These antibiotics are available in your local feed store.
In summary, having an avian vet take a look at your flock would be ideal. Antibiotic administration isn't unreasonable although I'm suspicious that you're dealing with infectious bronchitis which can't be addressed with antibiotics. Please respond with further questions or concerns if you wish.