I was going to say calcium as well, waterdog is right though those kinds of issues usually show up on lower foliage first. Seeing that your 400w was only 5 to 6 inches, is leading me to believe that was your issue. Hope it clears up!Check out some mag deficiency pics and threads. That may be an issue. If you think that may be the cause try a very light foiler spray of epsom salts. It usually effects the lower leaves and not new growth though. Good luck!