So with the pH test kit form API, I tested water and got the result of pH 6.0 on 3rd October 2016 as below video
pH water test after five days of using limestone, it increase to 6.5 pH on 8th Oct 2016
12 December 2016 - Update
I was worried due to rainy season water may turn acidic. Looks like the limestones doing it job pretty well.