Fertilizer build up on s/h medium question

[Becca]

I have a few orchids, out of the many I have potted in s/h medium that continues to have fertilizer build up on the PrimeAgra medium. Some of the build up looks white and some looks kinda like a white crystaline look. I can even see the build up on the roots that are near the top of the medium and many of the roots have died and I am assuming from the build up. I have always flushed my pots with RO water every 4th watering since I have been using RO water, but before I just had city water...yuck. Now I have even stopped using fertilizer with these particular ones in an attempt to flush the pots even more....and I usually flush multiple times....I just can't seem to get the white buildup off my s/h medium or the plants.....should I prepare a different batch of s/h and repot these plants? I am stumped and not sure what else to do.