Here is some reading: about air sacculitis:
http://www.poultrydvm.com/condition/airsacculitis
https://www.mypetchicken.com/backyard-chickens/chicken-help/All-about-Airsacculitis-disease-H387.aspx
Some articles on MG...
Where are you located? Get a necropsy by the state vet if you lose another. That will diagnose the respiratory disease and if there is a secondary infection. Since you saw some improvement with
Tylan, it may be MG, but E.coli can cause air sacculitis during an MG or other bacterial respiratory...