Hi, sorry to hear about your shrimp, just popping in with some general advice. With shrimp, your Nitrate (NO3) should be 0, although 10 shouldn't be a huge problem and wouldn't have been the cause of this - it could be indicative of another problem though. KH looks high to me, it shouldn't get up to 6, I wouldn't think. GH and pH are fine, maybe the pH might be a little low, though that shouldn't be a big issue. Shrimp will eat dead things, rather rapidly, so you won't always see bodies. I'm surprised that an older shrimp would die first though, rather than shrimplets. That's all I can offer at this point unfortunately, but I wish you the best with identifying the problem and a solution.
-Crabby