i've acutely found odd pH changes in my water bottles.

I let them all sit for at least 24 hours at about 30 C
The water is all in 4L mil jugs (but ones that never held milk)

So if i pH my water to 6.4 and use it. a few days later i test it again the pH often goes up above 7. I find this very odd. Is it related to the plastic? is plastic basic or something. or could this be due to the reasons above:
-acids reacting slowly
-pH of water just changing with time as blue bear said
harris7 Reviewed by harris7 on . How come the Ph will change? I have noticed that when I mix my water and nutes I will test the Ph. and bring it to desired measurement. The next day when I test it again I notice it drops a couple points. I check cal on the Oakton tester Pen and its perfect. I've read on the forums to always check and re-check the Ph again and again. Does the PhUp/Down just disappear and disolve in the water? should I even bother to set the Ph of my water a day ahead of using it,or before i leave it to sit for a few days? I use a One Rating: 5