Here is my interpretation of how humboldtii may be able to be grown in bromeliads tanks in captivity. I've repeated this basic setup with and without medium many times over the years, there is always a small amount of loose sphagnum in the water tank thou. I just have much better growth with even a tiny amount of sphagnum. I'm found that growing humboldtii without medium in just an inch or so of water leads to growths that just will not support themselves and fall over with any movement. So I've started adding tree fern fibers to give it something to grab onto. If my overall health holds out I'm planning on building a brom/humboldtii vivarium this spring.