I potted zygopetalum in 1/4 inch lava rock and that is working well for them. I am watering them with tap water (it is hard water) and flushing the pots well at least once a week. They do dry out a little in between but not for very long. I fertilize now and then with a liquid organic fertilizer or the leftover hydroponics fertilizer. I did keep them inside for the summer so that they could enjoy the air-conditioning.