I use a light mix, coco fiber, pearlite (medium grade) (50/50) and I tossed in a bag of happy frog soil last time...

Now, here is another issue, this was taken from colostate(dot)edu under one of the co-ops studies:

Second, Hyponex All Purpose Potting Soil and Green Charm resulted in poor plant growth. Both of these media contain sedge peat, a fine particle material that tests to a low porosity. There simply isn't enough space within these media for the water and air that plant roots need to grow successfully. Black Gold All Organic Potting Mix resulted in significantly less plant growth in one out of two years.

Here is your third problem:

Hyponex by Scotts 2 cu ft Potting Soil:

Feeds for up to 4 months
Helps grow bigger plants with more blooms (versus unfed)
Contains sphagnum peat moss and perlite to retain water and improve drainage

The soil your using has nutrients in it, nutrients your plants do not need at this time, but in amounts that can fry them.

And fourth -

On top of your soil having nutrients and causing issues with plant growth, your adding MORE nutrients to a plant that shouldn't even be seeing ANY yet. My first shot of nutes isn't until they get put in their 1g pots (started in solo cups til root bound)

Fifth and Finally....
Your vegging under HPS (you shouldn't be) AND it's too close for seedlings. So not only are you burning the roots, your burning the tips. If you don't pull them out of that soil ASAP at LEAST stop feeding them nutes, and never pour your nutrient enriched water over the plant, the nutrients are for the roots, not the leaves.

Thanks for posting pics - and thats why we have people fill out the form, I was able to pull all this info about your soil

Take care of those plants quickly by simply watering until the bucket is full 3x that will sufficiently flush the nutrients you've already added. However it won't remove the time released nutrients from the soil. (You can see on my first grow log how quickly the wrong soil will kill your plants)