Using limestone for GH isn't a stable method of maintaining GH.
Every time you do a water change, GH will drop and the limestone will slowly increase.
Also doesn't limestone increase KH?
You said your pH runs at 6.5 but could be lower. If you have a KH of 2, there's likely too much waste in your tank lowering the pH.
Neocaridinas don't do well under 7.0 pH