Unhealthy looking plants even though seemingly near optimal conditions... Please help me out,

Magnesium deficiency

how often do you water looks like you maybe overwatering do you let them dry out before you water them

It says in your diary 18" away and a 400 or 600? Watt. Maybe a bit close?

I don't know your light, intensity or distance to plants but this could be a possible cause. Maybe too near. I've just this week had a similar problem and because it affected the whole plant and not just the tops, I couldn't see it being that but I was told it could be so I did back off a bit. Both distance and intensity and it perked up within hours. Try backing off a few inch and check in an hour or two and see if the leaves have picked up at all. By next day hopefully they are praying

Something is off in the feeds and maybe the medium. Purple stems is a P and Mg issues, you need to confirm your PH is on point, that the EC is the correct concentration and that that the medium does not have a buildup of salts in it. You might need a flush with correct PH and then a balanced feed to offset the damage done, ....[][][].... Good Luck!