Fairly healthy looking plant, do you know what's going on with those yellow bits on the leaves? You'll probably want to address whatever is causing that before you switch to 12/12.
We're all friendly around here & love as much detail and as many pics as possible on what everyone has going on.
Looks like a smart pot you have there, what size is it?
What medium are you using Soil? Coco? any extra stuff you've added to it like perlite etc?
What nutrients are you using, if any?
What is your feeding/watering schedule?
Are you checking pH? If so, what is it going in when you wanter, and what is the runoff pH?
Tent temps & RH?
Magnetic or digital ballast?
How high is your light? 400W should be about 35cm above your plant