If they are acting fine, not acting sick at all, moving and eating well, IMO, there is no reason to be treating them for an illness. The man who sold those chicks was taking advantage of a Newbie. Those chicks are at best perhaps 5 weeks old. They are old enough to be out from under a heat lamp. Some stated have a law that chicks must be 8 weeks old in order to be sold in groups of less than 6. I have a feeling that is why the man was misleading you. You can give them a plug of sod from an untreated lawn to help them build their immunity. Also, check out fermented feed. That is an other great way of improving the health and immune system of a flock. There's a link at the bottom of my signature.