The APF EI salts are safe for shrimp using the APF mix recipe, I've let my tank go 3 weeks without a water change and taken the TDS from 150 to near 400, no RCS problems, I have also overdosed 300ml of micro (not APF but similar) into my tank accidentally, didn't kill the shrimp.
I have also sprayed hydrogen peroxide at 6% concentration on my bogwood to help rid it of some BBA, using a supposed safe limit of 0.025 per Litre I can dose 1.5ml to the tank over 24hrs, last time I used it I did 1.5ml in less than 2 minutes (spot treated, 10 squirts from a small spray bottle that does 0.15ml per squirt), more than 50% of the water was out of the tank at the time so everything got a good dosing, fish and shrimp were fine.
I have seen shrimp exhausted by being chased and nipped at by inquisitive small fish, they don't always live to fight another day, the hunt is continual until the fish go to sleep for the night.
I have also seen hungry RCS attack CRS that have just moulted and tried to pull the brain out the back of the carapace, looks like a piggy back but it's not really, especially post moult. I lost four female CRS to male CRS this way in the space of a few days when hunger outweighed the desire to mate.
So if you can discount that lot above then it's likely your water. From what I have read (haven't seen) is that there can be moulting problems when the tank temp is high and the water mineral content is high (body grows faster within a more mineralised shell and are crushed to death from within because they can't moult), if the water change water lowers the KH it can trigger a moult. If it's not that then there is something else present in the water that is harming them over time and they eventually succumb to its effects. Have you seen any of the RCS moult, have seen any of their castings?
