Continuous problems with everything. (HELP!)

Our indoor garden had a horrible case of spider mites, but we were able to salvage some plants and keep them healthy and move 3 of our ATF into flower. Everything was going well until 2 weeks into flower. One plants started severely wilting down and its lower leaves started to yellow. I figured either overnute, or salt buildup so i flushed the plant with clean water. The next day the yellowing reached midway on the plant. I found my Ph meter was off so they were being watered with 5.1 rather than 6.2. I recalibrated the meter and adjusted the Ph. This worked and perked up the plant for 12 hours, until it started wilting again. They are in a 1 gallon netted buckets with rockwool and expanded clay pellets inside 5 gal buckets @ 1300 PPM under 2 1000w HPS/ML bulbs with plenty of airflow. They are about 2'6". Sorry for no pics for the time being. going to link some later.

Oh also its just this one plant. the other 2 are fine. From what i read it's either Fulsarium Wilt, verc>(?) wilt, or underwatering. I've searched my Grow Bible and Garden saver and cant find anything. I need to get back on track but keep having roadblocks.

Sorry for the wall of unorganized info. ><

if you can possibly check out the root system, might have root rot through over watering, roots should be a healthy white colour any tinge and you might have a problem ,has this tends to spread.,