Day 8 of Flower and yellowing on all new growth.

I have 1 plant in a 7 gal pot with supersoil. Put it under 1 600w hps about 24"+ and started notice yellowing at around day 5.
Just yesterday I gave it some PureBlend Pro Veg formula, Im thinking it just needs some N because of the fast growth?

Temps are ranging from 75 with lights on to 65-70 light off. Also water has been with a ph of 6.5-6.8.

So any suggestions?
Also having doubts about this PH tester I bought, its a cheaper digital one off ebay, so going to be buying a more expensive one tomorrow. Hoping it wasnt the PH!!!

Here is a link to my PH tester

I have a ph tester that look similar to that one. Take it to a local hydro store and they'll probably calibrate it for free. I always have perfect looking beautiful plants until i flip to 12/12. Flower is definately where the fun starts..