Purina Start 'n' Grow medicated comes in a burgundy-red 25- or 50-lb bag. Check at the different feed 'n' seed stores in your area and read the tags at the bottom of the bag. You want to give the little ones at least 18% protein but not layer pellets (too much calcium hurts little kidneys).
The medication is a coccidiostat.
I'm no expert but opted to go the non-medicated route with the chickies and so I use the non-medicated Start 'n' Grow that comes in the yellow bag. If they were a large flock I'd do medicated but not with just a few backyard chickens.
It's up to you to decide if going "natural" means something to you or not. In my case, I preferred to try "natural."
I prefer to let them become immune to disease naturally. They will free range when they grow up.
The following things were to boost their immunity:
1. Indoor rearing on straw in a baby brooder or cardboard box for the first couple weeks. This helped keep 'em warm and out of drafts also.
2. Mix a tiny amount of livestock vitamin-electrolyte into their water especially when stresses like box changes happen. Just a tiny pinch to a gallon of water. I checked the label and this doesn't have antibiotics or hormones but DOES have more vitamins than Pedialyte.
3. Yogurt time!! Once or twice a day, they get some plain Greek yogurt to give beneficial bacteria to their tummies. When they were little I gave it to them on my fingers but now they peck pretty hard so they get it in a dish.
4. Pine shaving litter.
If they start acting sick I will switch to medicated feed but knock on wood I haven't lost any. I'm more worried about predators than disease.
These were born on September 13 and are big enough now to give the ol' stink-eye to the camera...