Jefferson let me clarify something here. If your ppm goes down then while it IS an indication that your plants are eating it is NOT what you want to go by as far as ppms go.

The norm is more for the number to rise after a few days but either way you don't really know exactly WHAT the plant is eating so frequent rez changes and additions are still very important. What I'm saying is that your ppms going down is not necessarily a good thing. It's more of an indication of deficienies in your solution than plant health.