I use EC when measuring my nuits. I use ionic and this is what they use too. I do not know what the equivalent is for PPM.
As i said before I know it has not taken up any nuits because the level has remained the same. This is true no matter which way I measure. The pH is always 5.8-6.2 usually 6.0.
I think you might be right about the roots being the problem because the waterfarm top feed system I am using has allowed the root to poke through on the edges which is why its easy for them to get damaged when we are filling up the bucket.