Hey GPS! The reservoir is an 18 gallon rubber maid tote, but only filled to 12.5 gallons. They normally go through about 2.5-3gallons a week. When I fill them up, I add fresh
nutrient water to them. I mix up another 3 gallons and dump it in the reservoir. I do this because (I think) I watch the EC go down as well so when I put a new 3 gallon nutrient mixture in it gets bumped up.
I top I it off like this for about 2 weeks but on the third week, they get all new 12.5 gallons changed out.