Questions, what were the temps, what type of light were they getting b4 u added 400hps, and the food u gave them, was that the same ppm as you gave then b4 the new light? Without knowing all this, I would guess a conbination of factors; u probbily had then under floros so a bit of lum shock, plus the food was to hot and mabe the temp added to the issues aswell. (sometimes the food u feed can make them more sensitive to the light/temp but not enough to burn it by itself. so then u add more light and its enough to hurt the plants.)
Good news is that I have had plants come back to life that looked worse. If you dont have to much invested in these plants, I would jus start over,(It seems like plants i have stressed early on dont do as good in the long run) but if u are set on moving foward with these i would do a flush of water and mabe some kelp or vit B (low ppm). If temps r high, get that fixed asap, and untill they get better keep the light a good distance away, 2ft +.
I hope this helps untill someone like seamedian can chime in.