I’ve yet to give my week 2 veg any nutrients only water so it’s not burn ,
It appeared over the last two days wanted someone’s opinion before I watered them again.
Yup those brown spots are normally calcium def. I use it in every water and feed not a lot maybe .25-.5ml per litre normally when growing in Coco i add more as you need it. Also LED cause Magnesium def infant remember the science but if you google it you'll see