Rusty Brown Spots & Curled Leaves 3 Weeks Old HELP!!!!!!

I am growing Big Bang Auto Flower from Green House Seeds and 3 weeks in i am getting these burnt looking leaves my lights are 3 1/2 inches away and i am using 3 36 watt cfl's. I have seen a few gnats but i kill them as i see them and i spray the leaves with organic insecticide spray.

Got a couple things going on there.
What is your PH?
What kind of soil you in?
If your PH is OK, it's time to start feeding.
What is your temp by the leafs?

This could be either 2 things 1:Nute burn 2h imbalance.I'm banking on a lil of both but 2 definately

I am growing in miracle Grow potting mix and i am using miracle grow fertilizer 8-5-8.Temp at leave level is 22. PH 7

As i suspected your Ph is too high,lower it down a bit 6.5.-6.8 and use your fertilizer at 1/2 dosage since your plant is still kinda young.It's at the stage of when it can't handle it,
In my experience if you feed seedlings 1/8 strength you will have a better chance of not burning your plants at a few weeks in when u feed again.

your PH is high for sure

I wouldn't spray the leaves at all, if u only 3 weeks in the MG soil should have all the nutes you need so would not use any. The PH is a little high but would not say it should be an issue at this stage. Soil looks very wet, what's ure watering schedule, be carefull of over watering this will stop oxygen getting to root system and cause nutrient lock out. Would suggest feed of low dose of trace elements on next feed when the soil is dry and ready for watering.