I had this problem last year with chicks that I picked up from a breeder locally,
TSC chicks, and my own hatched chicks. They would look fine, eat and drink, act a little off and then be dead the next morning. I believe it was coccidiosis. When I treated them with corid the problem ended and the weak looking chicks perked right up.
A few things about this:
Chicks can have coccidiosis without showing bloody poop. There insides are being ravaged and they will die in a hurry.
Coccidiosis can be transmitted through the egg, so no matter how clean your brooder is, they can bring it with them.
Any stress can let the organism get the best of them and shipping must really give it a foothold.
Medicated feed is not strong enough to deal with the problem and Corid is necessary to help them overcome it.
Water for chicks should be warm so cold water does not shock their system. Particularly important for ducklings who drink more and are more at risk.
It helps to add vitamins and electrolytes to chick water for 2 or 3 days to help boost their nutrition and energy.
TSC sells it in powdered form and it is easy to mix.
Hope this helps some of you. I have been hatching and brooding for years, so this is based on real experience and a lot of research.