Can't figure it out (pics included)

My plants seem to be suffering from some sort of stress. I was thinking it could possibly be heat but being that it only gets to 81 at the canopy I don't think that's it. Could it maybe be Nutrient burn. Any insight would be greatly appreciated

Also how serious is it? Should I ditch this plant and start over? Here's a picture of the plant that sits right next to it and looks way better

It's nute burn flush well and feed very lightly!

Ok. Should I wait a couple of days to feed it or feed it right after flushing.