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.
Hempfester Reviewed by Hempfester on . Leaf problem I grow indoor on Special Mix Light soil , no fertilizer during veg state (4 weeks). I've only added a slow release buffer-tablet and some Bacto I use plain water ph 7.8 and cannot measure EC The plant is grown from seed and is 4 weeks old. Lighting source and distance from plant: PL-L 440W, distance 1-2 inch Air temperature 22/27 Relative humidity 45-50 % Lighting schedule 12/12 Only this one shows the problem other plants (10) are not affected The stains are more orange irl then on Rating: 5