I posted this earlier in this thread, but the simple answer is that you can’t effectively measure the pH of purified water.
Leaving the water out exposed it to CO2 and as noted above, with purified water small changes can make a big difference.
Okay thanks for clearing that up. My tank is still dry but when I start to fill it up, when I mix the salt with this water should I expect to see normal pH levels? Same with topping up?