i just had the same problem in my res. i do drain to waste so my pH is always dead nuts on. last week it started dropping into the high 4's. wtf? so i emptied it and used 3 bottles of h2o2 on it and some really hot water. vacuumed it out and refilled and all is back to normal.
and i use the max dose of Zone too.
i'm thinking about dropping an aquarium uv light in there and see what happens.
I wish it were my res. Its that 11gallon nutrient stock I just made. Thats like losing 1100 gallons of reservoir.
I feel like im killing the environment if I drain it. But I guess thats what they have water treatment plants for, right? Lol.
Sodium Benzoate the more im reading is something a lot of commercial hydroponic nutrient manufacturers use to keep their shit fresh.
Only works on stocks 100:1 or higher concentrated. 100-300mg/L of sodium benzoate.
I don't think it fixes problems. And from what Im reading these bacteria feed on the chelating agents and could possibly be producing harmful byproducts and at the very least unsuspending iron and zinc. So maybe I shouldn't keep using it.
And that its the DTPA and EDTA chelates that cause this.