Plants are slowly dying

[Viva La Vega]

So a couple of days ago I noticed some small discoloration on the first set of leaves of one of my seedlings. I tried to diagnose the problem with the short list of symptoms I had, but to no avail. Unfortunately the problem progressed and this morning I awoke to some pretty disturbing consequences. The bottom leaves that are effected look fried almost, but I've tested and retested my lighting setup. I know that the lights aren't burning them. And now the dark spots are spreading to the upper leaves. I water on a consistent basis (every 2-3 days). I also have a small gnat problem that has arisen recently. I plan on getting sand when I go pick up a thermometer tomorrow. Anyway, here is the grow information:

What Strain is it? Bagseed
How Many Plants? 3
Is it in Vegetative or Flowering Stage? Seedling/early veg
If in Vegetative Stage... How Long? Couple of days
Indoor or Outdoor? Indoor
Soil or Hydro? Soil
If Soil... What is in your Mix? Miracle-Gro Seedling Starter mix, perlite added
If Soil... What Size Pot? 20oz. cups
Size (Wattage) of Light? How Many? (1) 110w HO T5, (2) 23w CFL
Is it Air Cooled? Cooled by small fan
Temperature of Room/Cabinet? unknown - no thermometer
PH of Medium or Reservoir? unknown - no ph meter
Any Pests? gnats...
How Often are you Watering? 2-3 days
Type and Strength of Fertilizers used? none given yet

All of these pictures are of the most severely affected plant. My other 2 aren't nearly as concerning.

I have no clue what could be causing the issue. Does anyone have an idea of what it could be?

 

[Antics]

It looks like a transplant would be a good start.

I would avoid Miracle Grow in the future, in the past too many people have had problems with it.

What is the age of the plant from when the seed broke soil?

Thermometers and Hygrometers are about $3, definitely try to pick one up when you can. They usually come in one package together.

PH meters are extremely important as well. Since your plant is young, it most likely isn't a nutrient problem, as most plants go 3-4 weeks before needing nutes. So it's probably a PH issue.

This is where the meter comes in handy, we need to know what your PH is, so we know how to adjust it.

Sierra Natural Science makes amazing natural pesticides for gnats, and other pests. I highly suggest their products.
And in the same breath, I strongly advise you not to use any chemical pesticides on your Cannabis. There's no way to know how those chemicals will affect your central nervous system once ingested.