I'm so sorry your having to go through this! I would defiently try another round like you said of the corid and tylan. However if what they have is viral the antibotic won't help that but it WILL prevent a secondary infection. If what ever going on is viral then the antibotic can help fight off a secondary infection but won't fight the actual viral illness. I just went through this in my own flock. I lost all but one bird. It was devastating!! I tried baytril injectable, tylan 50 injectable, tylamox, I tube fed but still I lost them. I can only conclude it was viral and that is why the antibotics didn't touch it. But I would still recommend giving the antibotic to prevent any secondary infections that go along with viral infections. My heart goes out to you!!! Again I'm so sorry your going through this. I know how you feel. All we can do is try. I'm so sorry your loosing your birds. I am sending prayers your way. I hope things get better for you.