get Tylan 50 injectable, or Tylan powder for the water..
I prefer the injectable because it works quicker and they get the proper dose.
Tylan can be found at
TSC..in the livestock section.
your birds sound pretty sick..the sooner you get them on antibiotic, the better.
as soon as you get a nice day..air out the coop..clean if you can, and replace with new bedding.
what bedding are you using?
give poultry vitamins
give extra protein (canned beef cat food, or kitten feed is an option, or eggs, or grower or gamebird feed mixed with layer.)
make up warm mash for mornings..
water cooked oatmeal
feed
cooked egg
plain yogurt
unsweetened applesauce.
make crumbly not soupy.
protect birds from draft.
is the coop open or enclosed?
if you're able to safely put a warming light in the coop..that might help.
make sure it is not close to roosts, high up, and any cords are safely attached and hidden.