lower leaves yellow

hi im a first time grower im six weeks into first grow,i had it 24/7 light now just gone to 12/12 the plant is just over 2 foot tall,but some of the lower leaves are turning yellow,its in biobizz allmix witch has allready got loads of fert in it so i have been a bit cautious adding more im useing biobizz grow and bloom do u think its nitrogen it needs?i have added more but 2ml in 2 litres i got some pics but camera is crap soz any info would be great help,some leaves have yellow tips too i ph water and i let tap water sit open for 48 hours,its under cfl 300 watt with halid not sure what else to say.please helppppp. thank you. pics added.also barneys blue cheese is the strain.

What is the run off pH?

Wet

hi wet the ph run off is 6.4 to 6.8 i got testin strips so carnt be 100% on it but its light green i colour so all good?.cheers nickty