Seedlings slightly yellowing

I am growing in promix organic blend soil less medium. Given less than quarter dose of a root enhancer. Other than that only distilled water.

I am growing in a 4x3x6 tent, with a 400w mh/GPS lights.
Plenty of ventilation to keep temps down. But can't keep humidity above 50.

Planted already germed seeds on the 10th and all sprouted by the 12th
Just distilled water then when the biggest had its 3rd set of true leaves they started to turn a light green with yellowish edges and slightly twisting. I thought maybe a N def. so that's when I gave the quarter dose of root enhancer(4-10-3), but havent had time to see the results. ust wanted some more experienced advice.
 
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