2 week old chick passed during the night 💔

We hatched 15 heritage Australorps and all 15 have been very healthy and happy. We did have to assist 2 with hatching.

Last night one chick passed during the night. It had a relatively full, squishy crop. Butt area was clean. I checked for coccidiosis. They’re all fine. I was thinking sour crop potentially??

I know that it’s rather common to lose one or two, but I’d still like to know a possible cause. All of them were growing well and were very active.

I’m also thinking maybe the heating plate was a little bit too cold last night and maybe they piled on top of each other and suffocated one??? The weather has unexpectedly gotten colder. QLD Australia for an idea. It’s not usually cold here

What can I get them that isn’t antibiotics or other harsh synthetic medicine?

I’ll give them some ACV in their water today

Photos for attention. It’s not the chick we lost