Discoloration of the leaves

I'm growing three white widow plants in 20 gallon planters with miracle grow soil under a 1000W HPS bulb. They're about three-four weeks along.

Unfortunately the soil was contaminated with gnats. I've done my best to exterminate them without harming the plants, covering the soil with steel wool, letting the soil dry out and turning the grow fan on higher than I'd like it, but they're still maintaining a small presence, albeit not as pervasive as they were a week or so ago.

So far I haven't given the plants any nutrients. Then two days ago I noticed these yellow spots appearing on the third and fourth sets of true leaves -- on all three plants. -- Any ideas what's causing it and how to remedy it?

get rid of miracle grow,, buy some decent nutrients... could be simply water spots..

check the under sides of leaves with a good magnifying glass, 30x or better,, hope its not pest damage

It kinda looks like light burn

over nutes. your plant isnt using all the nutes its gettng and has too much so its trying to expell them thru its leaves. but with too many nutes it will burn your leaves like that. miracle grow soil is horrible for growing in because it is always infested with gnat or other insects. Premium soil companies pasteurize their soil to kill off bugs. miracle grow is also full of nutes already so you cant control what your giving to your plant.

So, as I said I haven't given the plants any nutrients, so I'm not sure how it could be too many nutrients. I also don't see how getting rid of the soil is feasible at this point without shocking the root system. Any other suggestions?

Miracle gro may or may not be the culprit. Whatever it is doesn't look too serious. Best just to wait it out and see if it progresses. Almost looks like something got spilled on them.