I’ve got some dark patches on a few leaves which...

Did you pre charge the coco with nutrients? Looks you have deficiency or not enough oxygen to the roots. Coco is nothing like soil, you cannot let it dry must stay moist (but not wet). In the seedling stage small amounts of water to the base of plants daily a few cups will do. Then once the roots are established you can start to saturate the pot. At first it may take 2 days to make use of enough water to need to water again.