leaves have random light brown/yellow spots. (pics)

hi, these plants are from seed, they are northern lights, growing indoors, they sprouted on the 18th of march, for 2 or 3 days now there are a few leaves that look like this:


Attachment 284075Attachment 284076Attachment 284077


can someone tell me what this could be from? they are fed with flora nova at low strength and they are growing in miracle grow soil. they are watered every 3-4 days depending on how dry the pot is. any help would be apreciated