This is my first time trying to grow. I'm about 3 weeks in from seed germination. I'm having problems with the lower leaves of some of my plants starting to yellow. See this pictures below:

I already pinched off a couple leaves that started to yellow, because they weren't getting light anyways, but now another leaf, which is getting some light is starting to yellow.

Looking at problem charts, this could be a Nitrogen or Magnesium deficiency? I'm watering with pretty "hard" tap water, so I'm guessing this would have a lot of Magnesium in it.

Soil is what I could find locally from big box stores: Miracle Gro Performance Organics container mix (0.19-0.03-0.03 N-P-K) mixed with a 1:32 ratio of Jobe's Organics Fruit and Citrus fertilizer (3-5-5 NPK). I guess that would make the soil something like a 0.28-0.19-0.19 NPK value. Are these numbers too small?

I'm trying to grow "organically" so I haven't been feeding them any nutrients.

I water when the first inch or so of soil is completely dry; once every 2-3 days.

Light is a 135w (from wall) blurple LED, in a 36"x20" tent, with 190CFM exhaust fan.

I'm not sure what the soil PH is. I will get a PH meter and NPK test kit in a couple days.