Burn or deficiency? In need of help!

I'm baffled. I've been vegging for 6 weeks and my problem is getting worse.
It's tough to describe. The middle and bottom leaves are yellowing and developing small splotchy brown spots throughout the leaf. The tips are turning black also. There are no signs of this problem on the top leaves. The pictures make it clearer.
My PH is at about 7. I thought it was nute burn so I flushed. When that didn't help I have it a half-dose of FF Big. Now it seems to be getting worse. I thought for sure it was potassium deficiency but now I have no clue.
I would really appreciate some advice.

get your nutrient or flush water into the 6's, but checking the medium's PH is crucial aswell, you can check this by testing the runoff, this will let you know whats going on down there, you may have a lockout, if this is your first flush and your at 6 weeks veg you may have salts built up throwing your PH out of wack, the only thing you can do is give plain water and monitor, it looks like N is the problem. it wouldnt suprise me if your runoff was much different from your water