Any ideal what this is I've noticed it a few ...

What type of tester are you using to show the composition of your substrate? If you don't mind sharing. Thanks for the awesome advice guys.

I'm going to agree with @NobodysBuds with the mylar creating a hot spot. By not having it stretched out flat you can create hot spots that act like a magnifying glass and burn the leaf. Happy Growing

Some great answers here :)

Magnesium deficiency. Problem: A light green or yellow coloring will begin to show on the veins and edges of the lower & older leaves – this is one of the classic signs of cannabis magnesium deficiency. You may also see red stems. Magnesium is a mobile nutrient, which means that the plant can move it from old leaves to new leaves. Cannabis magnesium deficiency - yellowing in between veins on older leaves If you don’t react to it promptly, a cannabis magnesium deficiency can spiral out of control and cause your plant to lose a lot of lower leaves quickly. The plant will pull magnesium out of older leaves and bring them to the newer leaves. That’s why a magnesium deficiency usually appears towards the bottom of the plant and on older, less important leaves. The edges of the leaves may become yellow or bright green and may start feeling crispy to the touch. This crispiness around the edges is different from nutrient burn, which does not lighten the margins inside the leaves.

dial back nutes a bit. tip burn present elsewhere but no clawing, so can't be far off. high ppm, so less likely any of the symptoms related to a deficiency that can do similar things. Assuming you provide "enough" ca from any soil amendments and nutes tap water has some too. (based on diary info, seems unlikely you left Ca out.) p or k tox, best guess based on ppm, nutes used etc... could be lockout of Ca due to either. K should be 200-ish ppm p 50-ish. ballpark as ppl tend to have different substrate pH.. but should be about right for 6.5. Can eliminate this as possibility based on your more indepth knowledge of what you have fed throughout life of plant. One other possiblity due to the mylar i saw... is that leaf by chance in a hot spot for light? the lack of clawing makes me 2nd guess.