How old are they? Age can make a difference as to what may be ailing them. Also, are they on the ground? If so, have they ever been wormed? Have you inspected them for lice/mites? How are their stools? Do the crops appear to be full at night or are they feeling skinny?
Antibiotic use can cause something called "blood dyscracia" which in simpler terms is that the ABx cause the production of new blood cells to slow down and may even destroy existing red blood cells. = anemia
Brewer's Yeast is good in humans, I don't know for sure re: chickens. I would cook up some liver and feed them small amounts daily for a few days to help give them the nutrients they need to rebuild blood.
I hope this is all that is worrying you chicks. It is fixable.
6 months,,,they have been wormed and I sprinkled flea powder on them and treated for scaly leg mites too,,,,,,I am watching the stools now but nothing so far in the last hour,,,,Their crops were empty at night but now they are eating a little ,more because I am really pushing the food,,,, and peeperkeeper i think That sounds plausible,,,,I will try some liver,,,and brewers yeast cant hurt