Deja, take about 5 gallons of your house water and use some pH down to adjust the pH to around 5.7. Then check the pH every few hours and see how long it stays at 5.7. At this point, don't add anything to the water except the pH down.

Report the results after 12 hours.

The water should creep back up to around 8.0. We want to see how quickly that happens to determine if your tap water is useable.