Have you spoken with the breeder? She may have some insight into what's going on. The antibiotic that your feed store employee recommended, lincomycin/spectinomycin is for chicks up to 7 days old. Tylan, Gallimycin (erythromycin), and oxytetracycline are good for mycoplasma, and can treat coryza as well, but sulfa drugs like Di-Methox or sulfadimethoxine are maybe a little better. Here's a link about the LS50: http://www.qcsupply.com/media/pdf/540925_Label.pdf