I'm not seeing what you're seeing, I'm afraid... but no matter - they look strong and healthy! Pre-flower in autos usually comes in week 3 or 4. And to answer a question you had in your comment on the diary, yes, you SHOULD be concerned with the PH of anything you give your plants! Certain nutrients aren't absorbed unless the PH is correct. You should strive to maintain a range between 6.0 and 6.5 - 6.2 or 6.3 is ideal.
Good luck!