The sodium lighting makes it impossible to gauge color, but I'd say it looks like calcium or magnesium deficiency. Are they kind of pale green?

Assuming that they're pale, it sort of looks like an all-around nutrient deficiency (deficient on everything). If you flushed to 400 ppm but then didn't follow up with another feeding for a week, that could have been enough to do it. I'd do a foliar feeding with epsom salt in case it's Mg only. Your 800 ppm feeding should green them up pretty quick if they were just hungry.

Here's a reference photo of calcium deficiency from the web: