1 Blue Dream Velvet shrimp are dying and I have no idea why

[Mr Krabs]

Amonia 0
Nitrite 0
nitrate 0 (recent water change)
GH 275PPM
PH 8


i got them a little over a week ago.
one died one day when the tank PH was much higher and the GH was much higher.
then I used distilled water to bring the PH down and it stayed at around 8. I also brought the hardness from 400ppm to 275 ppm over the course of a few days.
after said few days
i was JUST looking at one of the shrimp. alive and well. come back 5 minutes later. dead.
I've raised cherry shrimp in this tank NO problems. no special treatment. not one death.
I even have cherry babies that were born in the tank before i even got the Blue Dream Velvet shrimp. they survived all the changes no problem. they do just fine.
what the heck is happening to my little buddies!?

 

[Chest Rockwell]

Well RCS are much hardier than a lot of shrimp. I have had the same problem when I've added Blackberry shrimp, Candy Cane, and Orange Sakura. I have seem to keep one of each alive so far for about 1-2 months. I recently started adding calcium to the tank either by breaking off a piece from a turtle block or mixing some Reptical in during water changes. I don't measuring anything but the basics and pH. My water is about 7.6 or so. I also have shells in my shrimp tank too.

 

[Autumnsky]

Do you happen to know your KH?

Also do any of the shrimp have any squiggly worm shapes inside of them? - do a search for horsehair worm in shrimp.

Do any seem to have one or more white dots around the head area? Bacterial infection would be possible.