Deficiency or Nute burn?

So 3 of my plants are showing 2 different problems. One has old leaves starting to turn yellow from the edges with being paper-y/dry. While the other 2 are showing signs of old leaves turning yellow from edges and the leaves look paper/dried, crumbles when I rub them.
It's a soil medium. I wasn't using much nute, some fish meal/molasses tea in 15 days and 2 times I used a NK soluble fert at 6.5:2.5 mix. The plants are 45 days old.
I wasn't pH adjusting my water which was 7.5. Have flushed them with 6.3pH plain water today.
Should I do anything more? Have taken 8 clones from them Which are all doing fine.

It may be Zinc, It is extremely toxic in excess zinc, I thought maybe iron but looks like zinc. it also interferes with Iron's ability to function properly and causes Iron deficiency. as you said 7.5 PH it is common in soils with a PH of 7 or more.try lowering the PH to 6.5 or less. avoid fertilizers that contain excessive amounts of Manganese,zinc,and copper which inhibit iron uptake. Good luck hope you get it all worked out.