I don’t think 6.8 is going to be a problem at all. 7 is neutral. 6.8 is mildly acidic, not alkaline. Most plants, not talking cannabis, prefer pH between 6.2 - 6.8. And that includes turf grass. Acid loving plants like blueberries, Rhododendrons, some oaks, many evergreens, etc. prefer more acidic soils. I’ve grown cannabis in soil or soil less mix for over 50 years and I never had plants suffer from a 6.8 pH. The pH affects the availability of nutrients and there shouldn’t be a problem with iron until the pH goes well over 7. I’m also not talking about hydroponics either.
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pH 6.8 is pretty near the frontier of the beginning of problems due to alkalinity:
and pH in soil could be higher and start having problems with Iron, Phosphorus, Magnesium and Calcium:
that’s why I suggested a pH runoff. It is common sense to allow some swings up and down on the pH while watering, always in the safe range. That allows the plant more possibilities to have all the nutrients range available.
If you’re too close to a limit, chances of being out of the safe zone are higher, that’s why I thought it was too high … 
Watering in I always try to keep it between 6.5 -6.8 as long as the meter is lying to me. When I double checked the meter it was about 1.5 low. so thats where my problem started, then me being and idiot trying to correct it gave it to much water.
I pick up a hanna soil ph meter and I am keep track of that now to. so this problem should not happen again. Everything is back on track and looking healthy again.
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I haven’t researched pH runoff dealing with cannabis but both of your graphs show the soil pH going over 7 and still nutrients were available. I often did soil tests for farmers, turf grass managers, gardeners, etc. Rarely did I see iron deficiency in soils that were 7.3 and lower. For many plants, like turf grass, you start to see obvious lack of nutrients and iron when you get over 7.5. I’ll have to look into runoff numbers.
pH runoff tells you what’s really going on in your soil. According to the OP last statement his pH meter was 1.5 low, so he must have been watering with 8 or higher pH, which would explain plant not assimilating those nutrients … 
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It’s true people check their soil pH but somehow don’t check the water. I’ve had my water fluctuate over the years. In the 90’s I had water that was in the mid 8’s. I simply added some white vinegar adjusting it down to the mid to high 6’s for every watering and feeding. Nowadays, it’s been checking out in the low 7’s. I wasn’t aware it could fluctuate that much. We had a few added houses nearby but not sure that would cause such a change?
I’m talking well water, btw, not town water. I’m not sure if town water is pH adjusted but I know it is tested for certain things (toxins) while wells are not tested usually for anything other than bacteria. Town water is also commonly treated with chlorine too to prevent bacteria issues. Certainly not good for growing.
What was the solution and issue finally?
Im curious what the ppm was of the runoff and smell.
With organics i leaned if theres a good balance of microbes, oxygen, and correct npk balance of feed that ph sticks round 7 or 6.7 with a probe run off about 6.5 testing with RO water.
So outdoors i didnt Ph my 400ppm hard 7.8ph water.
You can’t burn with organics but you can rot. You’ll smell it too if i smell ferts outdoors to me it means no need to feed yet.
Runoff of organic amended soil is usually high my semi organic runoff is 1200ppm in hempy pot.
So anyway that’s my shpeel, try to be at vpd, correct light, oxygen in soil, balanced amendment, light watering, etc seems like you figured it out already
The problem was that my ph meter was way out of calibration telling me the water going in was 6.8 but in reality it was around 5. I didn’t catch it early enough and thinking it was a deficiency I watered more then I usually do. so it was then over watered and having a ph problem.
I let it dry out and made a ewc tea and made sure the ph was right and the plant was looking good a couple days later.
Since going to organics I never water till run off. I just water a little then a bit more a day or 2 later. indoor im in 5 gallon pots sometime 3 gallons and I find I need to stay on top of the ph. Outdoor in a 15gallon pot it doesn’t make a difference the soil will buffer the ph.
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Thanks that reminds me to use the chemical kit measure ph time to time as the digital ones go off eventually especially forget to clean the probe
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Gonna have to agree with George on this one, for soil 6.2-6.5 is the cannabis sweet spot, less than 6 and upwards of 6.6-7 Id be getting concerned
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One thing I’ve done this year because of all the peat in the mix is watering the day before a feeding getting the soil completely saturated then scratching a ring around the trunk mounding up the grow bag walls and throwing my compost tea on it the following day. Eliminates the waste from peats hydrofoil and let’s the tea completely soak in
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