Deficiency or PH Issue?

Hey guys! New grower here working on a couple of plants. This is my Photo period Lemon OG Kush from DNA Genetics (Currently Day 15 from Seed). I noticed since about 3 days ago (Day 12) those bottom leaves are starting to get dark and splotchy.. Any ideas on what the issue may be?

I know I messed up the PH during the germination/sprout week but it did not show any issues. This is the 3rd week (Day 15) and I got it transplanted from a solo cup to a 2 gallon pot.

When transplanting, the root structure was great. No sign of root rot and fuzzy white roots. To note, I also have not fed them any water since transplanting this morning cause I'm trying to figure out the plan of attack to prevent this from spreading further.

Any idea on what this may be pointing at? And any suggestions to try and fix?

General Info:
Feeding: CloneX Solution 1/4 strength, Kelp Me Kelp You by Foxfarms 1/4 strength, Wholly Mackerel by Foxfarms 1/4 strength, Real Growers Recharge at full strength
General PH: 5.8-6.5
Grow Media: Coco Coir with Rapid Rooter for germination/starting.
Temps and RH: 75-82F and 50-65% RH
Lights: Spider Farmer SF-4000 @ 24 Inches away from canopy 40% Dimmer Light Setting, 18-6 Light Cycle

Any insight would be helpful!

Please let me know if I can add anymore information for you guys.

Also wondering if Foxfarms Bush Doctor Boomerang would help.

Thank you!

P.S. The grains that we see in the middle of the leaves are little Coco grains, not mites or anything, haven't seen any bugs or pests when looking in my handheld microscope.