Yellow leafs slow growth

Coco medium are not often pre-enriched in nutrients and when it's the case they are specifying it on the bag for a better marketing so yes your leaves are pale because they can't find anything serious to eat , in coco you need to feed straight from the start and with every feeding , and as coco medium is cal/mag free you'll have to bring some with every watering ; btw 1.5 L is too much , just water around the roots zone and check the soil by putting a finger inside to see if it's wet even if the surface is dry.