Your pH is slightly low, but your REAL problem is that your plants are freezing cold- are they sitting on a cold cement floor? Cold soil will show MUCH MUCH worse lockout symptoms than soil at the optimal 70'F. Also, you are running MG soil.

To keep from spreading:
Stabilize temps at NO LESS than 60'F night!
Get pots up off cold surfaces
Withhold water- let the pots get rather dry between waterings, and improve pot drainage/root zone oxygenation.
FLUSH to bring pH to 6.8 with tap water. You also probably have fertilizer buildup from running a pre-ferted dirt plus liquids, so the flush will take care of both problems.

But your big issue is temperature.