Why are they dying?

I did a water change on my tank yesterday, I matched the GH to 5, which is same as my tank and then added that and the rest was pure RO water from evaporation.
I came down today to find a few of my shrimp either dying or dead. I’m not sure why, as I tested the water before the change and this morning a nothing had changed significantly other than my TDS went from 150 to 140 or thereabouts.

What could be going wrong?

Thank you