You are in error, to put it kindly. Acidity in the mouth is what creates a bad situation. This is one of the reasons why it's necessary to become educated on a subject. My diabetic husband decided to fast back in the days before my first fast. THAT'S why I bought Dr. Joel Fuhrman's book "Fasting and Eating for Health". I was fearful for my husband. That book taught me a LOT before I even started.
I went to a dentist after a fast. My usual mouthful of tarter and plaque had NONE.
NO plaque NOR tarter. They were astonished.
That's one of the BIG differences between Water Fasting and juice fasting.