Water PH rising as it sits. Why?

I put some water in a cup and set the PH at 6.5 using down from up/down. I left it there for a few days to let it gas. I went to use it today and for some reason I decided to test the PH. To my surprise it has risen to 7.6. I thought maybe it was the instrument so I tested it with drops and sure enough the color was greenish rather than yellow. Note that I used both methods of testing when I set the PH to 6.5. Why would this happen?

As a side note, little bubbles had formed in the cup the day after I set the PH to 6.5.

@Bob31 @Myfriendis410 @Donaldj @70sChick

Ph adjusters are only for short term usage @Medforme

Let the water sit out 24-48 hours to evaporate the chlorine and then set the pH right before you use it.


Yes: one should never trust a solution that has been standing for more than 8 hours. What I have found is that as you adjust a given volume over time it tends to drift less.

Remember that ph stands for potential hydrogen and there is a constant exchange of hydrogen going on all the time. Hydrogen is chemically reactive to a lot of elements so it’s not surprising.

The bubbles were probably CO2.


Here is a great article on alkalinity and PH. It helped me understand what is likely happening.


@bob31 @Myfriendis410 - Thanks for the response. After reading the article, I wonder how much the water ph changes while it is in the soil.

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@Medforme it does change and you can take advantage of that to target specific nutrients. I usually vary my ph in from low acceptable to nominal (5.5 - 5.8) to take advantage of the change.

Are you using city water ? @Medforme
@bob31 and @Myfriendis410 got you here tho lol
City water should always be left to stand at least 24 hours
I would also ise a filter in it to help remove chlorine
Well water can change as well my water ph will be different depending on the time of year as well I have well water not city water
But I adjust when I use it never before :+1:
Happy growing


i know this post is madddddddddd old, but im having the same problem, im using tap water and when i changed my res it jumped up to 6.8 in two days in hydro. and needless to say. you could tell as soon as you opened the zipper. i regret changing the water. because the last one had stabilized beautifully. i went through and removed all of the grey wilting leaves. im hoping these girls will be strong enough to fight back. my blueberry seemed to be most effected (thats my favorite strain) D: i adjusted back down to 5.8 and reinforced, “if it ain’t broke, dont fix it” i also found a pollen sack on one of the stems but have yet to observe another one.

@Donaldj is a good resource on hydro grows fyi

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might actually be pythium now that ive investigated further

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