I agree about the appearance of a slight mag deficiency as well but potassium deficiency if you let it go to that level can mimic mag deficiency by causing yellowing in the leaf margins between the veins just like mag deficiency. Potassium deficiency is a tricky one to diagnose because it can also look like nutrient burn or the light too close. It's a multisymptom mimicing deficiency. Seeing him post that those plants have been vegging for 3 months I can almost guarantee that it's potassium. The soil is now partially depleted after the long veg time and he's only been feeding a 6-4-4 npk which is very low on both phosphorus and potassium compared to nitrogen. A bottle of big bloom fed fairly heavily should correct them. It has no nitrogen so it won't effect your current fed schedule like adding tigerbloom would. Tigerbloom is 4 nitro. big bloom is 0 nitro. You don't want to get nitro toxic just trying to boost your p&k.
By your pics it doesn't even look like it's on all the plants. It's more the plants in the intense part of the light. That would make sense they are burning up more nutrients.
I will give them a full feeding with the bloom in 2 days. The veg time was so long because I put alot of stress on the plants in the beginning plus I was only growing with the CFLs untill 2 weeks ago they have grown so much more already with the led. Do you have any other tips is the light a good distance how about the store bags as pots is that a waste of soil and space
I've seen people use poly bags that are a type of weave that breathe. I've heard they are the cheapest way to go if you want to discard your grow pot after but still have some of the aeration benefits of fabric pots. Fabric pots are far superior to plastic bags and pots. I'm sure you've saved a little cash using the bags but they aren't reusable so eventually a nicer pot would be cheaper even if it takes year and it will grow you more pot from a healthier plant. If you factor in increased yield a set of nice reusable smart pots or air pots will pay for themselves in one to two grows max. I like reusing my pots without cleaning them really well also. You transfer some of those beneficial soil organisms to the next crop instead of having to repopulate the pot each time. Adding some epsom salt to your routine wouldn't be a bad cheap idea. It would cover you if you did have some mag deficiencies and probably result in a healthier plant.
About an 18" light distance should be fine. This is always strain and garden dependent. The plants will tell you when the light is too close by curling up the serrations on the uppermost leaves along the whole outer edge. Sometimes called taco leaves. Watch for that condition. If you see it move the light up a few inches.
The next group of plants I am going to change to fabric pots. And do a better scrog this set up is difficult to work around and I didn't let the plants Stretch enough