have looked for it.no black dots dude.struggling with yellow leaves all of my plant for a while.add some nitro but couldnt fix it.Have a look under the leaves for black dots and as kmksrh21 said Nitrogen def... Cal/Mag de is prob the main reason mate good luck.
sounds like the same prob. dude.was just looking for epsom salt.let me know if it works.IMO your burnt tips are from the 400w, I have same problem... and also with bottom growth yellowing and eventually dying off has been happening, but it's only with lower growth, I added a high N food which helped slow it down, but didn't completely solve my problem, my next suggestion would be to try cal/mag or just some epsom salt... I just bought some epsom salt myself, but have yet to try it...