Sick chicks...please help....
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In the future, you need to quarantine ALL new birds in order to protect your flock.
I saw many people used a drop of Vet Rx to cure respiratory illness.
I am sorry I can't be more help. I also didn't know about quarantine, but I keep learning every day how important it is.
I personally would treat ALL, even those with no signs. Here is a link I found. Hope it helps!