Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Slow water flow through the creek, or else it may wash out whatever soil you use for the plants. Perhaps the plant pockets are not in the stream itself, but off to one side where a little water splashes in, not the full flow. I have had ferns to really well this way.
Many people use house plants for this. Many house plants come from tropical jungles, and are OK with a shallow substrate that does not have much soil in it. You might try the coarser lava rock (3/8") or orchid bark (also about 3/8-1/2"), and coir or whole sphagnum moss for epiphytes.
Another option is to prepare planting pockets then slip a mesh pot into them, and disguise it with whole sphagnum moss. Again, the substrate would be fairly coarse so as not to escape through the mesh pot. Get mesh pots in pond supply stores or on line. Clay (terra cotta) pots work well, too.
Depending on the base material, you could drill holes through wood or bark and make a basket sort of thing for epiphytes. Just enough whole sphagnum to hold them in place, then fishing line or polyester or nylon thread (cotton rots) threaded through the holes to secure it all to the wall.