That looks like a nitrogen issue to me..Im no expert..But if your using the same lights and same medium than its gonna go back to the lighting..These plants are simple but delicate...They can also be very durable..The more i look the more it looks like Cal-mag issue...Nitrogen deficiancy is on...
I also have the 4000k alto non coated CMH original design i use as well..i payed 25 or 30 bux for it off of ebay..This powerball has better cri than the rest...If you google the best CMH lights out there it will show the same exact same bulb i have only its 300-400 watts 2 pin bulb..
Yes...light burn actually gives the plant a sunburn so to say...And its yellowish to orange to brown then crisp..800 watt cob will def burn if not adjusted..I would set it at atleast 26-30 inches..maybe even higher than that..
It looks like it's a water issue Coco does not retain water as well as soil so it looks like you might be using a hps lighting.. I would say step up the watering and make sure you water them at least every day at that size because they're sticking up the water so fast it's it's a must to water...
Just get some gorrila glue adhesive and bond some velcro to the tarp.. you will have to kind of do a test run just to see where you will need to put the velcro exactly but yeah but the adhesive on the velcro and then the velcro to the tarp let that set for at least six to eight hours or maybe...