With coco, I start a very light feeding at 5 days old. I would feed it at 20% strength. Coco has absolutely no nutrients in it, and the seedling usually has enough stored energy for 4 to 7 days after germination. If you have an ec meter, mix you fertilizer solution to 1.0 to 1.2 ec. If you do not have an ec meter, get one, its vital for growing in coco.