Most likely you're dealing with a respiratory disease, not gapeworm. Duramycin 10 is mixed in water as you know. Sick birds usually wont drink, simply because they are too sick. I recommend that you purchase tylan 50 injectable and administer it orally to each sick pullet, that way you know they...
You wonder about the good of quarantine: It depends how long new birds are in quarantine. Six weeks is recommended and stress plays a HUGE roll in exposing "otherwise hidden" respiratory diseases. There are some diseases that have incubation times greater than 6 weeks, but most of the common...