A good majority of the threads I found on here that talk about respiratory issues mention Tylan50 but I’m not familiar with any of it so here is a link of a post someone made:
https://www.backyardchickens.com/threads/treating-upper-respiratory-infections.755483/post-10710001
I wouldn’t cull any of the sick birds. It’s not their fault their previous owners treated them poorly. I know if you nurse them back to health they’ll be beautiful and love you that much more.
I know there are tons of threads that talk about how to treat respiratory issues. I’ll have to go...