Newb Question on yellowing leaves

Hi Everyone

This is my first grow attempt. I am currently growing 4 plants and they are 3 weeks old.

I can’t remember the name of the soil I used, but it was recommended from the hydroponics store.
I have a 3’ x 3’ grow tent, 900 Watt MARS II LED light, and am using Pure Blend Grow nutrients (following the recomm. of 15ml per gallon of water). Lights are on 18/6 right now.
I am currently using test strips for the pH, but am buying a ph and tds meter tomorrow as I don’t trust the strips. From the strips, pH appears to be around 6.5
I also have a small fan, set on low, to provide a slight breeze.
I originally had my light set 18” from the top of the plant but moved it up to 24” yesterday.
Two days ago I started noticing yellowing on 2 of the plants leaves. I am wondering if this is nitrogen deficiency or maybe burning from the lights.

I’ve attached pics of all 4 plants, Wondering if anyone can identify a possible cause?

Thanks

Welcome.
Plants look good for the most part.
They're a little small to be feeding full strength nutes.
If the soil has a good charge, plus you are feeding, you could throw the balance out of whack.
That yellowing on two of the plants could be too much of something locking out your K.
Definitely not an N deficiency. Very unlikely light burn.
Back off a little on the nutes. Half strength the most. Maybe just some PH'd water for a couple of waterings first.
Let it dry out good too.

The soil is loaded. I will back off on the nutrients for a bit and flush for the next two waterings.

Thanks for the advice, I really appreciate it.