A pH much higher than the tap water is almost always rocks or substrate. Try pulling out all rocks used as decor and see if that helps. Another thing to check is a sample of your substrate in a small container of tap water. By drawing two water samples and letting them sit side by side for a couple of days, one with the substrate in it, you will have what you need to test the substrate. If the pH of the two is not the same after a couple of days you have found the cause. The fizz test that so many people rely on is only a gross screening test. If you get a fizz, the item will very quickly drive pH but if you don't get a fizz, that does not mean the item is safe. It only means the item is worth investigating further.
