I agree about the root issue.
@nakes
The first thing I noticed is that you didn't mention the heat in the tent?
You did mention the extraction fan being out, that may play around with temps a bunch, but it will have a bigger impact on the RH, depending on your setup. Is the extraction fan only cooling down the light? If RH is a problem and the plants can't breathe properly it's gona be a problem.
Another big one, where the root problem becomes the leaf problem - Check on your rootzone temperature in correlation to your leaf surface temperature. Those HID lights are not enough to heat up a rootzone, they give off nice heat, but once the cold settles in the soil the light will basically just heat up the surface of the stuff it hits, like your leaves. Giving you big contrast in the temp around your roots and on your leaf surface. Then the heat from the light becomes more of an issue than a good thing.
How big are the plants? If you talking bout throwing them away, you gona replace them? Just asking, because they clones I know they photos and photos can be turned around at the grave. I like making recoveries instead of hitting the restart button, cause I learn more that way. If it was a bunch of autos I couldn't really help you much and if it was a bunch of different phenos from a seed pop I'd say no way of throwing them out, but if they still small clones and you got replacements I'd say feed them to the worms. if they big plants it's way too little of a issue to be thinking bout throwing them out.
Another reason I ask how big they are - 250w mh in a 1.2 tent is actually a very good combo, but at what distance is the light hanging in correlation to how big the plants are? they're clones so I can't go by age or say "seedling" stage, but clones go through a sensitive stage too when they small.
I'd like to add, smaller rootzones will generally be a bit colder than bigger ones. Eg. - a control measure would be to take both a 10L pot of soil or water and a 100L pot of soil or water, heat both to the same temperature on the surface and internal, take them both out in the cold, the 10L soil/water would have cooled down much quicker than the 100L pot. If you have a plant growing in there and you got some life going, some movement, gas exchanges and all that then it creates some more warmth for itself. Smaller pots typically don't work in a living soil way, big living soil beds have nice internal temperature that it self regulates like it does with PH and EC and all that, but with smaller seperate pots need something external to keep the roots warm to keep up with the 23 - 28°C range you want above soil.
How do you grow - Organic? Synthetic? What kinda feed you use and all that stuff?