Sounds like your tap water is very basic and probably "hard" (filled with minerals) like mine. You may want to make up your tank water with a percentage of distilled or reverse osmosis water. If your tap water is like mine you will definitely want to use RO/distilled water if you need to replace evaporated water between water changes. The minerals become more concentrated as water evaporates, and adding more tap water adds more minerals.
Typically the minerals that make tap water basic have what is called "buffering capacity" the ability to absorb acidic inputs without letting the pH go down. What this means is that you can add a lot of pH lowering chemicals with no effect, and suddenly, when the buffering capacity of the water is exhausted, the next amount of chemicals added will lower the pH a lot, killing fish. I am mentioning this because I have made this mistake, and, not understanding buffering, I kept adding bigger doses of pH down and crashed my pH when the buffer ran out. Now I thin my hard/basic water with RO or distilled but I don't try to modify it further with pH changing chemicals.