I run coco...here are a few pointers and thoughts.
The tinfoil...I get it...but, it has a tendency to cause hot spots....you'd be better off by just painting the area in flat white paint...cheap and easy.
Your pots look too big for those plants if you're running coco. Too large of a pot in comparison to the plant, can make managing the water in your coco difficult because the plant can't move that much water on a daily basis, which keeps you from watering daily; without issues, and watering daily is what you want. Remember don't water with plain water...ever, and don't let your coco dry out.
Did you wash and buffer your coco before you used it? If not, at least in my experience, you will have pH issues and Mg issues almost off the bat (part of what you're seeing)
Your plants look like they're short on K, but I would highly doubt it considering their size and state of veg. Most likely tied up in the pH
All signs point to a pH issue...remember 5.8 is your sweet spot. Be sure you're feeding some form of Ca and Mg on a daily basis, unless you're running pretty hard water and in that case I'd have it (Ca and Mg) on hand the minute you had an issue.
Good luck.