My first grow!;)

That my friend is magnesium deficiency nothing to be worried about you can correct this by simply adding some CalMg to your nutrient solution . Once you add the CalMg solution to your mix you will see that the yellowing will stop and it will go back to a lush green :) And based on what I see in your diary that's all your missing everything else seems to be on point and well done ! You can get CalMg supplements at any hydroponics store or nutrient retailer . Sometimes the plants just don't have enough of it in the Veg or Bloom solution so its always good to add a little extra . The other symptoms you will see for this deficiency is the stems turning purple that is Calcium deficiency and should be treated the same way as the yellowing in your leaves you see now by simply adding CalMg supplement in your nutrient mix . It will not imbalance your mix so you dont have to worry about nutrient burns , usually the bottles will tell you to add 2ML per litre of nutrient mix . I hope this information is helpful in restoring your plants to extra kiss ass you well on your way to getting good medicine just a simple fix and you will be rocking and rolling ! -Happy Growing!