Respiratory infection

Hi

I bought some new hens the other day  and only noticed one had rattle in it's throat the next day. Now they all have it. I gave them apple cider vinegar in the water and garlic in the feed yet it's gotten worse. The vets say they don't have experience with chickens.

Could I use Dac Tylosin 10% in the water? It's primarily for pigeons yet I hear for chickens too.

I fear mycoplasma yet there is no smell, discharge or foamy eyes, just rattles and sneezes.

I believe it's based around Tylan.

The place that sold them went out of business and were selling chickens off cheaply, no pun intended. Only one appeared to be ill. I had mixed them with my established flock.