Perplexed

I had some old seeds from some plants I bred a few years ago and wanted to find a certain phenotype thats really mango pineapple. I just threw them straight into 12/12 from seed to find it and was gonna reveg it once I found it and make it a mother. The issue Im having is that theres a few plants that are just burnt up without even having a single dose of fertilizer. They are in roots 707 with no extra additives. Its not a hot soil and all other plants are showing no symptoms and are healthy. Im using tap water ph'ed to mid 6. Ive checked my meter accuracy and its on. I just dont get it. Makes no sense why just these few are fucked. They have been under a h.o. t5 until now but today im switching on the hps. Temps are a little high in the tent but not enough to cause these issues. Any ideas whats up? Shitty phone, shitty pics.

 

Weird. Ive never had a problem before. Thanks man. It was my first thought but then I was like nahhhhh, I have never burnt a seedling in it before and googled it and couldnt find where others had issues with it. Ive trashed like 15 males from that tent in the last week and those were all perfect, of course its only females that have burnt up lol.

 

I do have some soil left, the problem I guess is I have no ppm meter. I have not needed one in the last 8 years or so since i have been organic. In fact the last time i used roots was about 8 years ago and have only used my own organic soil outdoors. Im not use to growing indoors anymore. Its weird for me to even ph anything. I bought my ph meter to check the water because i just moved into an apartment while im searching for a new house. I hadnt used one of those in 8 years either. I will do a slurry and check ph, and next time im in town ill pick up a ppm meter. I hate growing indoors haha. Outdoors nature just takes care everything.