I meant to reply last night, but got distracted...
I think it's nute burn on all the plants, unfortunately. It's probably the Promix nutes. I'm guessing since you stated it could feed for quite some time that it has 'time-release' chemical nutes which are just too strong for the seedlings. I'm not too sure what you should do.. If it were me, and I wanted to try to save them, I would give them a really good flush, let them dry out most of the way, and then transplant into something like Promix (no nutes) or other soiless medium without any nutes. Maybe even try to dig off a lot of the medium at the top that doesn't have roots. I don't know though... You could definitely go organic, just don't start with any/many nutes in your mix next time!