What is this?

600w MH LST grow 18/6 schedule Miracle grow potting mix, 5 gal pots

What are these spots on my leaves?

Looks like a cal/mag issue to me! Any nutrients and how close to the light are the tops?

yeah def looks like a magnesium def, try some epsom salts or get some calmag and see what happens from there.

The lights are 16-20 inches away from the tops of the plants. The leaves dont show any signs of heat stress.. I check it regularly to ensure this.

I havent used any nutrients / plant food in my grow so far. It just Miracle grow potting mix and water at this point.

But this last one is a little different. It doesn't have the spotted or yellow/lightly colored leaves. It does have yellow tips on most of the leaves though. Nute burn? Just use PH balanced water to correct?

I'm somewhat new myself. That last picture is happening to my ak-48. I can't be positive but I think personally is nutrient burn. And agreed on the others. Looks like magnesium problem.