Roberta, WaterWitchin, thank you very much Our tap water contain only 70-90 ppm. But for my pleurothalides I use distillate water.
Most of my friends who love D.cuthbertsonii grow them in New Zealand moss on the south window is constantly wet. And they grow well.
But I know one person who keeps them dry, gradually accustoming them to dryness, watering them after drying the substrate. In high temperature. And according to him, they grow well with him.