Looks like a ph issue, nutrient overdose and possibly overwatering or spraying too often.

What are coco balls, and why did you add them?

Are they foliar fed, or sprayed with anything? (this includes any ferts, ammendments or additives)

Did you just switch them to 12/12, and did they look like that already? (4 weeks old?)

A two-part bloom fert...?

How are you testing the ph?