Yield and quality depend on many things but without a healthy rootzone ph, nothing else matters. Often times people increase or decrease nutrient strength because they see "nutrient burn" or "deficiencies" when the cause of those issues is rootzone ph.
Buildup of nutrients in the rootzone causes the ph to drop. Underfeeding causes the ph to climb. Any time you see an issue with a plant you should first check your runoff ph and ec.
A runoff check is done by capturing the last bit of runoff, say the last 10%, and measuring the ec and ph of that runoff. This needs to be done with a full strength nutrient solution adjusted to your desired feed ph. Without this you have no baseline to see a difference. If you were to feed RO or tap, of course the runoff ph and ec will change because there is no solid ph or ec value to the water. The higher the ec, the more stable the ph. You want to feed the exact same solution you have been feeding.
Most often the runoff ph comes out low. The ph of the runoff is the most accurate as the ph of the water flowing through the rootzone will change faster than it will grab excess nutrients. When the ph is lower than input we know there is a buildup and the ph needs to be brought back up. The best way to do this is to flush heavily with 6.4 ph tap water. Tap water is very alkaline and helps lower the acidity in the rootzone. Resume feeding 24 hours later. Do the next runoff test after 2 feedings. This allows you to get an accurate reading that is not diluted and affected by the tap water. We have now used 2 methods at once to increase the rootzone ph. The tap water adds alkalinity and the plant's nitrate absorption also helps to increase the ph. If you still find your ph coming out low, lower your feed strength and increase runoff.
If you find the ph of your runoff coming out high, you need to increase your feed strength and increase drybacks. Increasing both of these will stack ec in the rootzone and drop your ph. Continue to monitor your runoff so you can adjust your feed strength and drybacks when the ph has corrected.
We always suggest using the Bluelab Guardian to check ph and ec.