newbie with deficiency problem

I have has had no problems with this plant till now. I watered on sunday and i am getting ready to water again but i noticed that my newew leaves are turning yellow but the veins seem to still be green. i think it might be iron but i just want to see what you guys might think. here is some info

strain- unknown feminized seed
light- 600 watt hps
medium- jiffy pop and perlite ph 6.9
container- 3 gal.
water- distilled 6.5
nutes- master a 4-0-3 master b 1-5-6 1ml per l upping to 2ml per l next week
i am also using cal-mag plus



any advice or suggestions would greatly be appreciated

i almost forgot the plant is 25 days old