Sorry to break it to you, but yup, you are going to die. Part and parcel of that whole being mortal bit. :P
Anyways, your water should be perfectly fine to drink, and that's not what's going to kill you. Also, about all the Brita filters do is remove organic impurities and chlorine (they just run it through a charcoal insert).
It's worth letting your tap water sit out for a day or two, and then try testing it. I'm not sure what all causes it, but sometimes pH and such can change after sitting around for a bit.
I'm not certain about the rock - most of the concern about rocks is whether they are carbonate (limestone, marble, dolomite, etc.), which will affect the pH and the KH (maybe GH as well...) since you don't seem to have a difference twixt tap and tank on GH or KH, I don't know if the rock is necessarily affecting the pH.
The ammonia itself may be affecting pH, but I don't know how much of an effect it would have.
So, try testing some aged tap water, and compare that to your tank water, and let us know how it looks.