Seedling Help!

First time grower, doing a coco grow in a large grow cabinet vegging 21-3 under a 150w cfl. So after screwing up my first two seedlings with nute burn and overwatering, I decided to go back to basics for my next round of seedlings. No nutes, just plain ph'd (5. distilled water until 2 weeks. However, after starting out looking really healthy, my seedlings have suddenly started to die after 6 days. I have only watered 3 times, when the cup felt light and the coco was dry an inch down.

About a day ago, it started to show yellow along the spine of the leaf. Now, see pics. The temp has been in the high 70's, so I don't think it is heat damage. The PH of the runoff has been around 6.8, which I thought was all right. Is it a deficiency? I doubt I'll be able to save her, but for the next attempts I want to correct my mistakes.

Any help appreciated, thanks.

Do you have a fan going on the seedlings?
Your Perlite is getting discolored like the heat from your lights is cooking it.
Leaf temp is diff. than room temp.
Try misting it a few times every few hours for a few days, and if you're using a 150 watter, try backing it off to about 8" away.
Seedlings that small don't need as much light as they will in a couple weeks.
Also, your 6.8 runoff is high for coco..........