so if i'm reading correctly, you gave them 18 hrs light for 3 weeks (your vegetative stage) then threw them into 12/12 at the start of week 4? If that's the case i'll have to disagree with weedenhanced, there shouldn't be that kind of yellowing; also, the intervenal chlorosis with the green veins is usually a sign of a deficiency BUT that does not necessarily mean the answer is to add what's deficient ( in this case, I agree with Mr. Moonshine, you look like you're having a cal-mag issue).
How frequently are you watering / what is the soil condition when you water (is it damp etc.)?
What's your water analysis (how much calcium, magnesium etc. is in it from the tap, what's the pH)?
I'm not sure about your soil, and i know even less about your Bio Nova Mono Fert, but if you can get your hands on some magnesium sulfate (epsom salts) then 1-2 tablespoons per gallon of water, fed as a foliar spray, will help correct a magnesium deficiency.
Now, briefly, the questions above are to help you address some things that could be causing your plant to have trouble absorbing magnesium: water stress (over or under watering) is one of the most common issues people run into, and it gets misdiagnosed all the time. it is also possible that there is plenty of magnesium in the soil, but the ratio of calcium to magnesium is too high, which locks out the magnesium. Thus, foliar feeding the magnesium allows the plant to be "force fed" what it needs via the phyllosphere. There is also the issue of pH lockout. I don't have my chart in front of me, but 5.5 is on the low side for soil, and if your range of 5.5-6.5 indicates that your measuring a swing that big, then pH would seem also to be an issue for you, as pH swings and low pH can both cause lockouts and stress.
Hope all this helps, sorry for failing to convert units back to metric
be easy,
Dr.J20