Leaving water out overnight method for ph drop

I heard you can lower the pH levels by leaving a bucket of water out for a few days however I don't know if this works or if my ph meter is off. I left a pitcher of water out for 2 days first check it read 8.1 ..two days later it read 8.0 now I have Atlas pH Down however I'm running out am I doing this wrong

Oxygen will dissolve and can drop ph by a point or two (0.1-0.3ph). And you are losing oxygen from your water.

Just get some ph down, even if it drops a whole digit to 7 you are still above soil and hydro ranges.

As lil'Biscuit said, I don't think it will change it any substantial amount. I personally leave tap water out for 24 hours to allow the chlorine to dissipate and then I PH down using apple cider vinegar and measure it with my meter.

Phosphoric acid in pH down works well and very consistently.