Listen, if you've got bloody poop in a chick... it can be deadly fast!
Most likely suspect it coccidiosis and should be treated with Corid in the drinking water... no other water or vitamins should be offered during treatment.
You may want to do a drench dose for the chick you saw blood from if they aren't really drinking. Do treatment dose for ALL who are in the brooder. Keep water clean from droppings as that is the fastest way it spreads.
Can you post a pic of the poo or the chick? Sometimes over crowding is part of the problem. Or I have even had bedding that looked dryer on top than it really was underneath. Keeping it dry helps a ton.