Strongly suspect poisoning. Put any carbon you have in the tank now. Before even reading the rest of this post. If you don't have any carbon, drive to your closest fish supply store/Walmart whatever is open and I would buy the biggest container of high grade activated carbon you can and put it in stockings inside the tank, in the filter, anywhere you can fit it especially where there is flow. If you don't have filer bags, lady's stockings work great, just pour some in the toe and tie a knot, tie another know, pour more carbon in, repeat. Water changes *should* help as long as the toxin to the shrimp isn't originating in the source water. Agree to keep tank cool, air running, as much gas exchange at the surface as possible. My experiences in losing multiple shrimp is that they tend to keep dying, but all you can do is try to remove what is killing them. Fingers crossed for you.
Nothing good happens fast in an ecosystem.