The 2nd pic definitely looks like nute burn to me.and if it's the only thing you've changed,it's gotta be the soil,did you mix it with coco,or layer it under the coco? The plants don't look that old,but there's a chance the roots are in amongst the nutes,as soon as seedlings are up,they start sending roots straight down to the bottom of the pot.
I'd try some more seeds,but just in coco,see if that helps??
Hope you sort it out man