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I just went back to your original message. Day old chicks. I would definitely old off on the grit. I would reintroduce the grit at 10 days or so.
I think a table spoon per gallon will do. Also, try giving them some vegetables. Leafy greens are really good at that stage.
What exactly is the best meat chicken? I have heard a lot about the Cornish x
Go ahead and add the Corid to their water. If something is going around, it's best to treat all.