High pH out of tap, decreases with time
My water right out of the tap is a deep shade of purple on the API high range pH test, meaning it's somewhere between 8.4-8.8. I filled a bucket of water this morning, and ~8 hours later, it's now around 8.0-8.2. I have a bucket that's been sitting out for a few days, and it's settled down to 7.0-7.2.
I was wondering what could be causing this? I know that sometimes, tap water contains CO2 which lowers pH, causing it to rise once you let it sit for a day, but what causes the reverse of that? I don't have access to a KH, GH, or TDS test currently.