It sorta sounds like to me that maybe they got chilled during shipping. I had that happen to some Japanese Bantam chicks that I purchased and were shipped to me. I don't think a little antibiotics in their water would hurt but on the same hand I'm not so sure it will do anygood. I would make...
Do you know how long they had the chicks before selling and is it supposed to be a bantam or standard? Are you feeding medicated feed? You could try giving it a little sugar water for start. If that doesn't help then I would go back to the feed store for antibotics that you can put in the...