is it coco? it looks coco from here.if it is,6.7 ph is too off.this might be reason to burn spots.water a bigger area in your pots,try to get some runoff at the bottom,use only water,no nutes and ph set to 5.7 - 6 for a few days and start feeding again.this is for getting rid of excess salt and regulate the coco ph. it should be somewhere between 5.6 - 6.2 . i prefer 6.1 for veg and 5.9 for flower but any spot should be good for you at the moment.