Take the "sick" chick out from your brooder and take ONE other one with it. Chicks can not be alone or they will drive you nuts.
Isolate the two for a few days on paper towels, chick starter, fresh water and be SURE they have a heat source that gives you 95 degrees F at the BOTTOM of the brooder.
Get some poly vi sol (NO IRON) and give the sick one just a drop. You can put electrolytes in, but not too much.
I would hold off on the antibiotics, it probably will not help anyhow because CRD, MG or MS are not bacterial.
hatcheries are not necessarilly MG free. There are many threads about this lately.
Bubbles and sneezing are an indication of a problem. Whether it is a disease or an environmental issue is to be determined.