Phal Root Problems

[Cerey]

Hi everyone,

I have two phalaenopsis who I've recently noticed are having some trouble with their roots. The tips of the majority of the roots are turning black, and some have just dried up and turned grayish, while some have what looks like dry gray spots on otherwise healthy green roots.
They are both in sphagnum moss mixed with hydroton, with a layer of hydroton at the bottom of the pots. They are on a heat mat. I let the moss get more on the dry side before I water. I bought both plants about 2 months ago. I had cleaned the media and replaced it with the new moss/hydroton media. One of the phals currently has two spikes and is forming buds. The plants have not grown new roots or leaves since I bought them.
What are your thoughts on this? Could it be root rot? Not enough oxygen to the roots? Staying wet too long?
I have a bag of hydroton so I can attempt semi-hydro as soon as new roots start to grow, but for now the plants are just there. Is there any way to promote the growths of new roots? I have a bottle of superthrive but I'm holding off on that since one of the plants is in spike.