New root die in S/H

[The Duchess]

Hi guys! I have a small problem with a phalaenopsis. Is in S/H from 09 June 2018 and a new rot die. First grow from a old root, but then stop growing and now is dying and I don't know why... Is become light green/gray... Other new roots seem OK, they growing... But this one I don't know what happend with her..
I use tap water 150-170 ppm, pH 7.50 and I fill the pot to the top when I water. I fertilize with 25 ppm.
I use pelgrom euro pebbles, I wash them how good I can, but steel they have some dust when I use them, water have a orange color from dust. Maybe this clay pebbles are not good for S/H? You have bad experience with clay pebbles who are not good for S/H?

First 2 photos are with that root who dying.