I am pretty sure this is either overwatering or lockout from getting too cold at night. hydro bucket drip system, 2 1000w hps on tracks, clay pellet medium, ppm 600, ph 5.8 to 6.3 watering twice a day, but so slow the drip reservoir lasts for up to 4 hours on each watering. Noticed it has gotten into high 50's at night.
Sounds like over watering. Try not watering for 24 hrs. Watch for improvement or getting worse. In the past for my reservoir lines and drip emitters can get build up and restrict flow from salt build up or even hard water if using tap water. If lines and emitters are old might be time to change out. This goes for the pump also but they can be cleaned. But with good flow your watering cycle should be much shorter than 4hrs. Like 15 minutes. Is your pump on a timer? How high are temps getting in the room? Do the leaves have any yellow and brown on them? Hard to tell with hps light.