I have had that too before. You're right it is from root activity and your reservoir. What I found to help is using airstones in the reservoir, and keeping ph steady between 5.8 and 6.2. The more volume of your reservoir, the more stable and predictable it will be. 6 gallons seems a little light for 8 plants. I use 5 gallons to my three pots. Get as many opinions as you can. Try asking for Zandor, he's always willing to help.