I have read that flushing with a mild nute solution is best,50 plants is alot to loose this far in,have you checked the ph of the run off to see what its at,are there enough holes in the bottom of your pots to let all the water out?it doesnt really matter at this point what nutes or how much you gave them if there burned as bad as it sounds,the only thing you can do now is hope that they bounce back,I would raise the light so its not to close for a few days,and are you useing tap water by chance,some places are useing chloramine in place of chlorine and its harder to remove it from the water just a thought because it happend to me