Need some help identifying this

So I got an issue with a majority of the plants in my veg box. At first I thought it was heat, checked the temp and it was 90 in there, so I got a fan on it and now it's staying steady at 78. But they started getting worse. Take a look at the pics. There's the yellowing and browning leaves, and the leaves are in general really droopy. They look sick. When I thought it was heat, I checked with the moisture meter and they were dry so I watered, but seeing how they got worse, I don't want to do ANYTHING now until I get a second opinion, because I really don't want to lose these by over-acting.

Any help, soon, would be appreciated.

(PS, in the process of pulling everything out and photographing and closely looking at them, I noticed one leaf on one plant with those dreaded two spotted blood suckers, and immediately jumped on top of them, so if you notice the spotted leaves on that, unless you think the entire thing is being caused by the light mite problem, I already know and it's taken care of)

Probably nute burn, possibly ph issues too.

First thing I would do is run some water through them. Check the ph of the clean water before you run it through. Collect some of the runoff water that drains out the bottom of the pot and check the ph and ppm of it. If it looks out of whack, there's your problem.

Looks hungry, what medium u in and what fretz u using?

small container for that size plant. Your stems are so purple there black, leaves are drooping similar to over watering.

I can't currently check PPM, just kinda getting into this. But I've been using RO water with a touch of my 8.1 pH tap water (about 1:10 ratio). Medium is 2:1 MG topsoil and perlite. As far as the container size goes, I started them as seedlings in these, the plan was to determine sex by now and weed out the males and clip clones from the females (obviously I would have clipped the clones before flowering) but then when I started having issues I scrapped pushing these into flowering. As far as pH issues goes, would it help them if I transplant them first? Get them out of this medium? And I don't think it's overwatering, because I originally thought it was, and I let them go pretty dry when I thought it was heat. For a while. Longer than I was comfortable with. Thought it was underwatering too. I'm just learning how to read these things.