Looks like they're burning. Stop all nutrients and flush (water with plain tap until you get about 20% coming out of the bottom). After a day, start introducing cana Coco at half strength and work up from there. Small plants need very little. Also, little ones don't need nearly as much light intensity (think Spring). I would have the 600w around 18" to 22" above the plant.
Oh man, I'm sorry! I mistakingly misdiagnosed your plant. You are seeing calcium and magnesium deficiency. It's easy to mistake that with nutrient burn. For growing in Coco, CalMag is necessary. I'm not familiar with the products you're using, but I would see if they have a website and see what their feedchart says. They should have one. I would bump it up to 1.5mil/litre and see if the newer growth looks okay. The damaged leaves won't repair themselves, so go by the top of the plants. New growth will naturally be light green, so go by the top few pair of nodes (branches off the main stem).
Using clay balls with coco. At your skill level.
With coco already being a medium which holds onto water alot and makes it heavy youd want something more like perlite for coco as its extremely light in weight.
Coco is already very airy. That's all you need. Here's the thing about doing something like this, how many experienced growers here @ RIU are mixing their Coco with Hydroton? If the answer is "none", then there's a reason, and you should follow by example. If it was beneficial for roots, a lot of growers would be mixing their medium like this. What you've done is inadvertantly created a science experiment in which you don't know if your grow medium will negatively effect the roots, and ultimately your plants. It might be fine, but you just don't know. For future grows, just follow what the established growers do.
Yeah, I wasn't really sure what was meant by "heavy"
Nah, not needed.
You bet! Most of us are here to help
Your plants will be fine, but I wouldn't add any more clay balls. Not that it could cause problems for your plants, but because you're doing something that will have little to no benefits.