Need the brain trust to put your heads together

[PLreef]

Slight freak out today. Went down to look at the girls after Work. The flagship plant of this grow has been growing like mad. Super bushy. Well yesterday I thought I was seeing a tiny amount of yellowing in the leaves. But today is pretty prevalent that the leaves are turning yellow. Lower leaves only.
So I added in 5 cups of fresh coast of maine soil and 2 to 3 cups of fresh earth worm casting, large shot (25 ml) of cal/mag. Watered at PH 6.51 and 290 ppm. I checked my run off PH in a few of the pots and i was surprised to see it come in at 7.18 to 7.35. Should I be concerned? Seems a tad high. I'm gonna look up what I can amend with to lower it a bit. Any of you pros might have a clue on what's going on?
Growing in Coast of Maine Stonington blend with 25% added perlite, dash of Epson salt and a dash of dolomite lime. 5 gallon 5 gallon pot, Growers Choice Roi 680e (680w, @60), Spectrum King SK402 (460w, @ 60%), 2 Optic 1XLs pushing 100w each at full power. Room temp hovers around 75°F the temp at the canopy hovers around 79°F to 81°F. Tons of air movement via tower fan and a 12" oscillating hurricane fan. Girls are just over 5 weeks old.
I threw in a few more pics of the rest of the grow.

 

[Gingerbeard]

If it's just the one plant, might it be a root issue?

 

[Rwise]

A dash of dolomite, this buffers PH up, what did you add to buffer down?
I use pelleted sulfur, not a lot, it is not fast but buffers the soil PH.

 

[PLreef]

This is the normal mix I've always done except that I used a different brand soil. I've always had an issue with PH being too low not high. I have some pelletized sulfur that I could top dress with. I think I'll wait it out till my next watering and see how they are looking before adding anything.