Your tap water is 8.4?
I have a hard time believing that... I suppose it's possible depending on the source of your water, but that's quite high.
The only conclusion I can draw from this, assuming your tap water is actually normal (6.8-7.4), is that your test solutions have gone bad. At least the pH bottles.
You might want to buy a cheap pH-only test kit at the store and test your tap water with it. If it still reads high, let us know.
I'm guessing you've already checked the expiration date on your API kit?