Leda, although the common name is 'cactus,' these plants come from the rain forest, so would never dry out in nature. There is high humidity, plants constantly guttating, prolific dew dripping every morning, rains often. They are epiphytic, meaning that they grow up in the canopy of trees, where the roots would mostly be in the air, with moisture flowing past often. So in a pot, a mix that is very chunky with air between the particles is what is needed so they don't rot while being watered often enough that they don't dry out.
I wonder if a north window is enough light?
