Yo man Im loving the big old ladies. I have some DWC monsters that are a little bit bigger than yours and they use about 3/4 of a gallon of water per day. I would imagine that your plants are sucking quite a bit too(maybe like a half a gallon or maybe more) Now if I dont add water at a couple of times a day to top it off, the ppm's get way too high and it starts to burn. Its just because the plants are so big and are sucking up some much water and nutrients, ppm and ph can get out of control pretty easily. PPM's are spiking when the res get low, thus overfertalizing for a little bit until you can dilute it.