Chicks getting sick - any idea what it is and what can help?

[KlinoClucks]

Hello! Just wondering whether anyone can help me figure out what is wrong with some of my chicks.
I hatched them in the incubator about three weeks ago. There are 15 of them, mostly Light Sussex. Well, one of my Light Sussex chicks started looking lethargic this morning, and didn't eat anything and hardly drank any water. All the other chicks looked fine at the time.
Now, a couple of hours later, she's getting worse, tail and wings drooping, eyes closed, and wobbly when she stands. She hasn't eaten anything yet that I'm aware of. I let the chicks out everyday into a safe little run outside next to the vegetable garden since they were nearly a week old, and only had them under lights at night. Yesterday, though, the weather was pretty wet and cold, so I left them under the lights all day. I'm wondering whether that might be a problem?
I've now put a thermometer into their box to see if the temperatures are off, but I've let all the chicks out so it isn't on at the moment, so I'll check when I put it back on tonight.
I've given the chicks that look concerning some ACV diluted with water, and will probably be heading out again soon to give them some more. Not sure what this could be. Anyone have any advice?
Thanks

 

[tigger19687]

I would give a drop of nutridrench instead of AVC.
If you are turning off the light when they are out and not turning it back on till you bring them in, that may be an issue. It takes a little while to warm back up.
Are they all feathered out completely?

 

[KlinoClucks]

Yes, they are all feathered out. Where would you get nutridrench? I live in Australia out of town, so not sure if I'll find it easily.
I started giving her some yougurt mixed with luke warm water yesterday, giving it to her every few hours. She's eating and drinking on her own today. Still not perky and running around, but eating and drinking. Still keeping an eye on the other chicks that look concerning. So far they haven't gotten worse, but they haven't gotten better either. All of them are eating and drinking, and are still eager for food when I fill their feeders.
Will keep watching and keep you updated.

 

[rosemarythyme]

If there's no Nutri-Drench available, do any of your feed stores have any sort of vitamin & electrolyte mixes? They're often in a dry powder form.

 

[tigger19687]

Somewhere on this site someone said a Kids vitamin liquid, one drop. Can't remember the name but it wouldn't matter with you in Austrailia
If you gave Gatorade, a drop of that or some sugar in the water/dropper

 

[KlinoClucks]

Okay, will definitly have a look. I think I have seen electrolytes in the feed store before, will have to go check again.
I stopped giving her yougurt yesterday, and she ate and drank just fine on her own. Today, though, she hasnt eaten or had anything to drink as far as I'm aware. Perhaps I should try the yougurt again until I get the electrolytes from the store?