Slow growth & stains - what can that be and what do I do?

Hello there!

I planted two seeds (Minigun by Seedsman) and one of them started showing some signs of slow growth and stains. After reading some info, I thought it could be Magnesium or Sulphur deficiency, but I’m not sure since it’s not recovering so well after I gave it a little dosis of Epsom Salt and AlgAMic by Biobizz.
I added pictures of its twin sister, germinated and planted on the same days, so you can compare growth rate.

Can you help me to get to know what is this problem and what to do about it?!

Strain: Minigun (AK47 Auto) by Seedsman
Pot size: 4L
Soil: Carolina Soil XVI + humus
Light: COB LED 200W
Tent: 40x40x120 Mylar
Germination: 7/5/2020
Sprouted: 7/7/2020
Deficiency Signs started: 7/13/2020
First feeding: 7/13/2020 (Flowermind Fertilizer 2ml+0.25/1L)
Epsom salt (1/4 tea spoon) + 1ml Algamic by Biobizz on 400ml water: 7/20/2020

From the first purple picture, it almost looks like the one had a light directly above it and too close.
If that was the case, being that young it will really stress them and cause major differences like you're seeing.

Or, could be some completely different.
Best of luck

I think overwatered, which is causing nutrients not to be available.

Generally speaking, when a plant this small is showing a 'deficiency' it's generally on account of pH. A plant this small isn't pulling huge amounts of anything from the soil, especially not micro nutrients. Dial in your pH and you'll most likely see the improvements you want. Good luck.

thanks a lot for your replies, @.Smoke, @Mrs. Weedstein and @CannaCountry . I think it can also be the strong light and because I didnt water the soil for almost 3 days... I measured the soil pH and it seems normal, but I'll check it again and wait some more days to see if the plant recovers. Appreciate your help.