Yellowing along edges of 1st true leaves

Hey guys

wandering if anyone could tell me what’s going on with these plants. Yellowing on two of my four plants. each plant only has it on one of its first set of true leaves.

details on setup.;

•4x4 tent

•430w quantum board led hanging at 26”

•380ppfd

•RO Water ph to 6.5

•built my own soil following coots mix

•day 20 of veg

•1 gallon air pots.

•watering every few days when soil is quite dry.

•humidity is controlled through an inkbird controller with a humidifier @ 70% rh

•temps are very consistent. 26 deg C day and night (78.8F). •oscillating fan above canopy

•ambient C02 420ppm

•ac infinity 6“ extraction fan coming on when temp hits 27 deg C (80.6F)

not exchanging a heap of air at the moment while the plants are small and the c02 is stable at 420ppm.

possibly phosphorus deficiency. I flushed them last night. ph of runoff after flush was 7 didnt have a reading of ph runoff before flush though. PPM of runoff after flush was 900 with three week old soil. I’m assuming that kind of normal.

im a complete rookie though so any advice would be much appreciated.