Franci, Crystallinum is actually highly epiphytic. In the wild it grows is shallow scree and detritus that tops karst, with the roots growing down the rocks into crevices. Overwatering an doverpotting is not good for Crystallinum, you are correct about that.
I really don;t think there is any 'BEST mix for Aroids' as a blanket statement. Each and every species should be researched to determine how it grows in nature, and then those conditions emulated as best as possible. Mineral substrate and lava rock or Aliflor (Leca, Hygroton etc) is good for lithophytes, moss or chunky loose mix is good for epiphytes, and potting mix blended in to add substance is good for terrestrials.
