A few things stick out to me:

1) 15L pots are pretty big for new clones in coco. Plants grown in coco like smaller containers compared to soil.

2) ph is way off. Should be between 5.5 and 6.2 with coco, many prefer in the 5.8-6.2 range.

3) Are your nutrients coco-specific? I'd check out Canna's site: About Coco | CANNA UK and make sure you are following there instructions.

4) Check your nutrient amounts according to Canna's schedule. From the pics it looks over-fertilized, but it could be related to your ph problem or 'contamination'.

I'd put them maybe a little farther away from that 600w, if you have some other lights like fluoros or cfl's, that would be even better, just to nurse them back to health. And find out what 'contamination' it was. Pests? disease?