Poorly 3 week old leghorn chick

[Parsnip]

Can anyone help. I have 16 3 week old leghorn chicks and one is very unwell today. The chick is listless and fluffed up a bit. When I picked it up its crop was squishy and it freely vommited a murky looking fluid. I massaged the crop and it vomitted freely until the crop felt empty.
My question is, is this sour crop, if so have I done the right thing, what causes it, and what else can I do. The chick still looks unwell.

Worried.

 

[Mrs.Puff]

Put Apple Cider Vinegar in the water. Have you by chance been giving them anything but chick feed without giving them grit? If they've been getting treats and no grit, they can't "chew" it very well in the crop, and it can get stuck in there.
Other than that, I don't know.