Don't do nothing for some days with your older plants and see if they recover...probably is only overwatering.
And don't change nothing to your young plants. They are shining.
Reiko is right...we need some LITFA
I'm with this guy. I've played with halogen a lot for the uv/fr/ir qualities. I doubt it was the problem as long as heat wasn't an issue.
I can attest to the leaf burn LEDs are capable of! That is some intense focused light right there. Kind of like cooking ants with a lens.
what ever you do give them some time to recover first.