A couple years ago some guru name LostGirl did a tutorial on how to calibrate the exact pen you have
Personally I’ve used this exact same pen for about 3 years now. They’re excellent pens for the $17 that you pay for them. However, they don’t seem to last more than a year. For the price, I normally buy three of them. I can usually get a couple of grows out of one and then I switch to a new one as calibration seems to fail.
Always have a backup pen to compare to.
If you ask me, it’s the pen. I also believe calibration would be the first thing to try to see if you got the same numbers.
I based this theory on process of elimination. The nutrients that you utilizing (Advanced Nutrients) typically will not give you any type of big salt build up as the nutrients are pretty light on salt. Also pH issues can also leave you with poor nutrient uptake by the plant but I would eliminate that thought as well because you’re utilizing the pH perfect Advance nutrient system. This system has never failed me nor giving me any pH issues. (I’ve been using Advanced Nutrients for over 4 years)
My recommendations
Buy the calibration solution for the ppm and also buy a backup pen. I firmly believe you’ll find your solution to the issue here. I would do this as soon as possible.
There’s many knockoffs of this pen. Most suck really bad. I always buy mine from the manufacturer on AliExpress. (This could take many weeks to get to you during the holidays)
This is the actual manufacturer of this pen
https://a.aliexpress.com/_mL9tym5
For now you can get this pen pretty much anywhere on Amazon. It should take care of your immediate problem. These pens on AliExpress usually drop down to about $15 and I’ll buy two or three of them. You have the link above.
Also be sure to buy the 1413 calibration solution and watch the video in the LostGirl link above to calibrate your EC
How do I fix my three plants
Unfortunately I strongly believe the problem lies within the PPM/EC calibration of your pen. This means we truly have no idea what the actual PPM of your runoff is for each of your girls. Once you have calibrated your new pen, and you have a backup pen, you’ll need to mix up some calmag water pH to 6.5 and simply feed your girls with it to get the true runoff.
If any of your girls PPM is super low you can then mix up a feed right away and run it through and it will not affect our watering as you just watered it.
This should take care of your issue