Had same issue with 2 of my plants on last grow. I flushed with Sledgehammer and then watered with nutrients and properly PHed water. It got things right back on track.
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The plant is at 47 days above soil so I am guessing I might have another 2 to 4 weeks before harvest. I donât think there is any way I could remove the plant from the Smart Pot at this point without damaging it. I am hoping my buds develop nicely in spite of most of my upper leaves looking bad.
I flushed with 14 gal of water and have added a total of 5 tablespoons of dolomite lime. The highest I got the PH was 6.2 and now it is back down to 6.0. That is watering with 7.0 water.
I know every thing that I have harvested (autos so far) most of the upper leaves are in pretty rough shape by the time I cut her down. Browned out and dry. Do what you can to see it through.
Do you think this is from the light? Dbrn32 thought perhaps my light was too close but I am at the recommended 18" from top of plant. I am afraid if I raise it my lower buds wonât get enough light.
Are you using FF nutrients.
Donât be pissed at the soil. This far in, most mediums start to acidify. That pH that is guaranteed is its starting pH. After months of growing, microbes and chemical processes tend to acidify the soil.
I recommend slurry testing.
Are you talking RO/Distilled? Tap water pHs just fine.
No, I was talking about tap water. I respect that everyone on here has some valuable experience they are willing to share with the rest of us. However, the general consensus is that it is not possible to accurately PH tap water. I can put my digital PH tester into tap water and get a reading but I have no idea if it is accurate.
I am about to create a new post regarding a reply I got from Fox Farm customer service. I will tag you.
I did do a slurry test. Results are in this post. They were not good. PH 5.3.
@smokysmurf thatâs my grow. It was 90 gallon but between 9 plants. It took the better part of the day. For what itâs worth, after the flush; I have seen EXPLOSIVE growth. They are 17 days old today and Iâm happy. They just got their first haircut this morning.
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That is incorrect info. Distilled and RO, yes. Not tap water. There is no arguing about this fact. 
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Iâm a first time grower and learning every day but when the majority tell me you canât PH plain water I tend to believe them. If you say I can that is great. Sometimes I donât know what to believe because there are so many different answers to the same question.
No majority is telling you that anywhere, I promise. You may be misreading info.
There are a lot of newer (and older) growers who may be giving incorrect advice. Definitely vet who you take advice from.
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Just for future reference:
You cannot accurately pH RO or Distilled water because there arenât enough ions to facilitate ion exchange.
In tap water, which contains many different chemicals, there are lots of ions, which allows for more potential cation exchange, which is what pH measures (Hydrogen exchange, specifically).
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I use tap and PH just fine. My tap is generally 5.4 âwell waterâ and adjusts to 6.5 easily.
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How does TDS or PPM affect the ability to PH tap water? My tap water is around 45 PPM. I do remember specifically a few people telling me I wonât get accurate PH reading with PPM so low.
Think of pH like a party. It measures how much social interaction there might be with our buddy Hydrogen. If the party isnât attended by anyone (RO/Distilled water), Hydrogen wonât have anybody to talk to, so there will be zero social interaction. If the party is filled with all sorts of people (Tap water), Hydrogen will interact like a social butterfly.
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Thatâs exactly it! TDS is what sets RO/Distilled from tap. That number tells me your tap must be filtered. Thatâs incredibly low. Most tap water is not that low.
So, you are generalizing an issue but itâs specific to your own tap water. See the problem?
So yes, your tap water is too low.