Help. Day 23 leaves turning yellow with grey spots

[Canada4crop]

Growing in promix hp with 20 percent perlite. Plant has been growing like crazy but yesterday some leaves started yellowing. The lower fan leaves especially. They are on day 23. Not sure if its overwatering (it's been taking about 4 days for the 5 gallon air pots to dry out). Or nitrogen deficiency? I've been feeding Advances nutriance ph perfect at about 1/4 strength. With a small amount of calmag at the last feed. About 400ppm total. Any ideas? Yellowing with a couple of grayish spots.

 

[bushmasterar15]

Looks like it might need to be bumped up to 1/2 strength feed. Not sure what the amount your using but when I did use Sensi Bloom A/B Hansbrick/Tangs schedule I was already at 2ml per Liter

 

[Waira]

no doubt, long term starved at 1/4 str.,... AN doesn't have very high NPK#'s to begin with...
they are starting to tap into the fans for what they can mobilize, ...not all the nutes can be translocated though! ....up the dosage ASAP-

Thanks @bushmasterar15 !