Cattleyas in desparate need of help!!

[PhalaeFunkadelica]

Hi,
I haven't written before, mainly because I couldn't figure out how to post a new thread (sad, I know), but I have some seemingly simple questions that I cannot seem to find the answers for onine:
I have two cattleyas in a terrarium. The humidity/temp is monitered, and is usually in the range of 60-80%, 60-70F. They are in plastic and ceramic pots, and I mist them often, usually once a day.
The problem I am experiencing is that, despite misting and watering, the roots still rot, with only a few good ones on each plant. I did have the plants in a northern windowsill, but they weren't getting enough light so I stuck them in the humid frog terrarium with full spectrum lighting. At first they seemed to adjust well, but now after about 2 months the new top growth seems to be turning brownish black...?
What's going on?? Am I doing something wrong? I have them in coco fiber, frankly because I live in a small town without proper orchid/plant suppliers. Is coco fiber okay?
Also, do I still need to fertilize them in the winter? I live in KS, and the days have been very cold the last few months making it difficult to maintain a warm environment.
Any input will help!
Thanks!

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