New Seedlings Are Yellowing

i have some new seedlings, and the new leaves they have been sprouting as of late are looking vary yellow, i thought it might be a N deficiency, so i bumped up the ppm from 300 to 600, they didnt get any nute burn but they are still yellow, any suggestions?

Give them time, keep up that nute level, make sure of the pH, and watch the new growth. New leaves are a lighter shade of green when shiny, and darken as the shine fades. Are you providing enough fresh air? As your plant increases its foliage, it needs more and more fresh air.

HTH