do YOU know whats wrong?

DAMAGE CONTROL !!!!!!!!

Stop all fertilizers, and nutrients.

The dirt looks very dry, how often are they being watered.

Flush the soil with clean distilled, ph ajusted, room temp water,

make sure there is lots of air circulating in there, as this will help blow the toxins out through the leaves (or whatever is left of the leaves).

Check plants for any bugs or infestations.

When did they start to turn for the worse???? did you do anything different.

 

If you flushed them good with clean water there is not much else to do after you have ruled out over/underwatering, good lighting, high temps, soil PH and good air circulation. Monitor all those things and keep your fingers crossed. Good luck!

 

It's obvious that the plants didn't always look like this , and it's seems to be happeneing to all the plants, so it must be a change in enviroment...you know, something that is effecting all the plants at the same time.
Either the grow medium, or the water. or the enviroment. maybe light cycle.
Sorry i can't be of more assistance. good luck hope they bounce back, it would be a shame to lose the lot, they look pretty tall..