Where are you located in the world?
In the U.S. "medicated feed" is generally chick starter and contains a minimal amount Amprol/Amprolium which is a Coccidiostat that treats Coccidiosis. It's not an antibiotic and would not treat symptoms of respiratory disease.
How much ventilation is in your coop?
How old are these birds?
What are the symptoms? Coughing, sneezing, mucous, facial swelling, bubbles in the eyes, etc....
Depending on what you are dealing with, you can try treating Symptoms with an antibiotic, this will not be a cure, but may be a way to control symptoms so you can get your birds into better health. Respiratory diseases are contagious and make birds carriers even when recovered, so finding out what they have is always a good idea. You risk any new birds you bring in becoming ill.
Tylosin/Tylan and Tiamulin are both used to treat symptoms of Mycoplasma. Sulfa antibiotics can be used to treat symptoms of Infectious Coryza. Infectious Bronchitis is a virus so has to run it's course, but Tylosin/Tylan may help clear up secondary infections.
Personally I would not use the DE at all. It can be an irritant to the respiratory system (yours and theirs) and possibly compounding the issues they already have.
The one that is sick, if you lose her, then sending the body for testing/necropsy so you can find out more would be helpful to you in finding the right treatment.
https://www.metzerfarms.com/poultry-labs.html
Lastly, any time you add something to their water - Acv, garlic, etc. provide fresh plain water too. Some birds will not drink very well and become dehydrated.
Here's info on diseases
https://edis.ifas.ufl.edu/publication/ps044