Based on the fact that you have low pH, your KH should be 0. This means you should be using RO/distilled water with GH only mineral if you aren't already. If you are, then something is leaching KH into the water... which could be rocks, substrate, or?
Unless of course, you want a higher pH, and you are intentionally adding KH into the column... if this is so, then each time you do a water change, you'll have fluctuating water parameters until the substrate can no longer absorb the KH out of the water column and then the pH will slowly rise. These fluctuating water parameters are not ideal for a shrimp tank, although amanos should, theoretically speaking, be fine.
What's the ammonia in tank?