HELP! New chickens are making old chickens sick!

I have 9 healthy, young hens/roosters and they've lived happily for almost a year together. I wanted to add more and got 12 silkies....2 died the first couple days and the rest looked very sick. Stupidly I put them directly in with my flock right away and now everyone is getting sick! Some are worse then others but symptoms are lethargy, most have one or both eyes closed...the eyes and nose have discharge which is all crusty....sneezing...some pant like its hard to breathe. I brought the new chickens home on Friday and started them on duramycin and Corix on Saturday....I have 4 water containers and ALL are treated with this mix...for the ones that I notice aren't drinking enough I assist by squirting a syringe in their mouth. Nobody seems to be getting worse but not really better either and the worse part is my favorite, healthy, young ones are now starting to show symptoms. It's been 5 days so maybe someone can suggest a different route. I'm not experienced with raising chickens and am still learning so please be clear with your instructions...

 

Does the tylan come with the syringe? Can I reuse syringes? Also how often and for how long do I give it to them?