Good advice from everyone here! I agree with feeding a 20% all flock or grower feed like Flock Raiser (my choice for years!). Make sure that the mill date on each bag of feed is within one month, and that you can use it up within two or three more weeks. It ages out! The feed store people may claim that it's good for six months, but not true!
Permethrin spray concentrate is easiest to use and by far the least expensive product for external parasites, and that fecal test run at your veterinarian's is cheaper than treating with the wrong product, or for non-existent intestinal worms. And Ivermectin is not approved for use in chickens, so avoid it.
If any birds actually get sick or die, spend for the necropsy to find out why, before investing in new birds. Some diseases are forever (Marek's) or require depopulating and three weeks without any birds (Mycoplasma) so be sure, and be careful!
Mary