@Tim Miller dude you don't need a soil ph tester (I've never needed one), your way over complicating this if you go down that road and spending cash for no reason...
the possible reason for the diff is that the soil is acting more like a hydro coco grow due to your amendments to the soil and your growing under LED's which we all know has a habit of making the lady's consume calcium and magnesium... one question I have is are the stems near the top of the plants turning a slight purple?
This is really nothing to much to worry about and is super easy to fix... (plus a good lesson in growing)
How would I go about fixing this if this was in my G'room -
first remove the leaves that are damaged by the diff. By dong this after the next few feeds you know If you have go on top of it. Those damaged leaves will not turn back to green... keep an eye on the new growth.
Now my next step is controversial... I would then do a mini flush through with a microbe tea just in case its a salt build up and a lock out. Flushing about the volume / two thirds of the pot or near about's (I don't Ph that water, don't want to kill those microbes)... I will measure the PPM's and the Ph of the run off from this flush, taking a sample from the start and a sample from the last drops coming out from the pot. This will give me a good idea of what going on in the medium. If my PPM's on the first sample comes out lower than going in I'm possibly under feeding. Vice versa if the PPM's are higher than going in you know your over feeding and may begetting a slat build up causing a possible lock out... your also looking to have a run off Ph of about 6.3.
To combat the Mag Diff I would then foilia spray for a couple of days with Epsom Salt (cheap as chips from any garden store) and the magic spray
Overgrow by opticfiola... a must have weapon for the G'room, fixes most small diff's on the spot and a great foilia feed for your system... (I'm sounding like an advert here)
On the next feed I may increase the cal mag by a few ml if i think I've been under feeding. If I've been over feeding I will knock back those PPM's by half to one third and slowly increase the PPM's over then next few feeds if needed (which for me is the next tank re fill)
If you do want to spend some cash I highly recommend that you put your money with the best of the best Ph pens by Blue Labs (most other brand are inferior). Also if you want to go down my road of the G'room Weed Nerd you need a PPM / E.C pen... my recommendation is get them separate, Blue labs does a 2 in one unit. Don't get that one, the Ph probe that come with it breaks easily at the cord. I've gone through 3 in one year (at about £70 each...ouch). Buy the pens.
It was me that was asking about the worm castings... reason is that I'm having a similar issue in my G'room with some clones and I'm pinpointing the cause. If we had the same brand it may have got me thinking with another avenue to look down. I do think the main reason for my Mag diff in my G'room is that I've change coco brands.
