The sure fire sign of coryza is the foul smelling exudate from the nostrils. Give the ooze a smell, and you'll know if you're dealing with Coryza.
All birds having contracted a respiratory disease (they do NOT get 'passing colds' like us) will remain infected for life and a carrier for life. All that Tylan, Sulmet, Gallimycin etc. do is to (temporarily) drive the symptoms underground. So does Vet Rx, if you get lucky with it.
In time of stress the disease can appear again full blown. No new birds should ever be housed with any bird(s) that has had any respiratory illness, since it too will be infected.
You might investigate CRD (MG) or IB as other diseases.
Then again you need to make sure the problem isn't just dusty conditions or ammonia build up, which can cause disease like symptoms. Good NOT to assume disease right away.
Much luck to you.
Finally, injectable antibiotics--if you go that route, some will not do so--are better to use than water soluble, because you KNOW that the bird is getting the proper dose; with water solubles, it is very diff. unless you want to risk syringe feeding them, to know that they are getting the proper amount of water in, in order to be truly medicated. This can save time and $
Peace and blessings