Need help, fast! No idea what’s happening.

I’m running coco dtw, week 5 of flower, strains are grape smash, northern lightsx willie Nelson and sournanadubx Wilson.
Have them on automated water, 5 times a day. Been feeding at about 1100ppms.
yesterday I noticed the Run off was extremely high. One plant was in the 2000s. The plants were perfectly fine at this time, just a few tiny rust spots which is what led me to check the runoff. So I flushed immediately and brought the run off down to 1300.Literally hours later this started happening. So I flushed down to 1000ppms and replaced the reservoir with 900ppm solution. Woke up today and things have gotten much worse. Here are pics. Humidity is 50% temps at canopy are 75-79.

Looks like a heat/light issue to me

Well I checked my run off ph and it was 4.8 so that is the problem. My ph pen wasn’t calibrated properly. I have ran about 10-15 gallons of 6.2 ph tap water through them. The ppms dropped to 300. Ph barely went up to 5.3. I ran 5 gallons of 7.5ph tap water through them and the runoff was around 5.8. So I mixed up some nutrients at 500ppm strength and ph of 6.5 and watered. Run off is 5.8 still so hopefully this fixes the problem. Will be checking the ph a few times a day. I also removed all the dead growth. The plants smelled like they were dying. Kind of sad. All this happened so fast. Before this problem they were absolutely thriving.