I think it's the low humidity that caused the burn mate

running at the lower end of the pH range will exaserbate the burn in a low humidity environment too.
If you have a fresh supply of clones, it would be more productive to start again, if not, run pH adjusted (5.8-6.0) water until you have healthy new growth. Keep the extra hunmidity efforts up too, and remove any fans that are directly on the plants.
San
