HELP ph issue with bucket system

[KingGhidorah]

Quick layout. I have a 20 bucket system with 1 res x 2 so 2 systems for a total of 40 buckets and 2 brains. Plants are under 10 1000 w sodiums. They were just flipped yesterday. Now before i go to bed every night, i check the ph. the last two nights though i've been having major ph dips in system 2. Ph check night before was 6.1 and when i woke up ph was 4.8 Ahhh! I topped it off and got it to 6.2. When i checked it before bed it was 5.9 and about 20 min ago it was 4.8 WTF!?

I topped off w/ reg tap water but had to get out before my timer hit. Lights off until 8 tonight and my green light ran out of batteries. My two questions are: 1. What could be causing this? 2. How long can my plants survive in low ph. Water change is tomorrow.

Any help would be greatly appreciated.


Couple more tidbits. There are sitting in 5 g buckets, hydroto, and this is my first time.