Yep, the Dragon Stone is causing that.
But apart from 250 TDS, which shouldn't be that high that it would kill shrimps off, there might be other factors.
Tell us exactly how you added the new shrimp. What was your acclimation process?
The next time you buy any new shrimp or fish, always, ALWAYS test the water that they come in to find out how wildly different it might be to the water you are introducing them to. If the difference is big, acclimation needs to be more than just floating the bag in the tank to get temps up to equilibrium - it needs to have water dripped into the bag until parameters match your tank water closely. That can take anywhere from 1hour to 12hours depending on the difference in the water parameters.
Dragonstones are great for hard water fish, but not so much for Tetras and shrimps. I know you have probably invested in those stones, so someone telling you not to use them might not sound like the best advise. Just don't throw them away, you might be able to use them in the future (like when your tank matures, and pH starts falling too low). But for now, try taking them out, and we can see what the tank parameters fall back to.