That new reagent sounds suspect to me, ditch it! Pick up a new PH test kit and use 2 drops of it's reagent in your DC in which you should have 4KH solution, you cannot use just DI water in your DC.

If you do not have a ref 4KH solution reach out to the guy or gal here for a 500ml bottle which will easily last for almost 2 years and thats factoring changing the DC weekly.