I'm not an expert in plant problems, but I'll chirp in anyway.

Could be magnesium deficiency...or another micro nutrient deficiency.

I would lower the Ph a bit and feed them with good nutes that contain a proper micro and macro nutrient mix for your growth stage. I'm sure someone with more expertise in plant problems will be along shortly to correct me if I'm wrong.

Start off slow on nutes at first if you have never given them any before.

I'm not familiar with your soil; does it come with slow release food already in it?

Also, I see you have your lights set to 12/12. Are you trying to force flower? If your plants are ready to flower, you should definitely be feeding them by now I would imagine.