You're making progress!
Once you finish the course of Corid, switch to plain fresh water.
You're feeding Nature's best Chick Starter? All are pullets?
I'd keep them on Chick Starter until they come into lay, then you can switch to a Layer feed. I would try to keep to an 18%protein layer feed if possible.
Alternatively, you can start feeding an All Flock/Flock Raiser feed now and just continue on that all their life.
Provide oyster shell free choice once they come into lay regardless of what you feed.