I always had some problem with D.filiformis, no matter what was the hybrid/ssp. But this year, I changed my formula, I grow them in LFS instead of the usual peat/sand mix. Since then, I haven't had any loss, but the dormancy hasn't begun yet (my major losses usually). Nevertheless, my leaves usually dry up easily, and I think that mosses can provide the lacking humidity, and a 'airier' substrate. I keep water at 1/2", and they have been both grown under HPS and fluorescent tubes. Note that the fluorecents tube were 6" away, and there was reflector all around on the walls. If you space coverred by your fluo is quite small, you can try to make an homemade one from foil. I did mine with mylar.