I'll copy what I posted elsewhere on this same issue:
High ppm actually slows plant growth down, since the plant doesn't drink as much, therefore not taking up the amount of calcium and other things it needs. Calcium is the big one though, and is uptaken totally passively, which means only through respiration. So if your humidity is too high, or your ppms too high, the plant won't respire enough and it can slow to almost nothing.
Try watering it down a bit, say to 600 or so, you might be surprised at how much they like it.