Thank you, burz, for the heads-up on the Growstone products. That's something that I'll want to try. I am checking to see if it's available, retail, in Edmonton. It sounds ideal in many respects, and should work well as a major constituent in non-soil media for orchids. It appears that the only negative aspect of this material is the silica dust in the dry product; one user points out that it will clog your drains, and he also recommends using a respirator while unpacking, as it's very bad for the lungs.
I have recently been using something similar to the GnatNix! for my regular-soil potted plants such as cacti. It's locally available, and the product is "zeolite." I simply remove the top layer of soil to the depth of about one inch (removing it to the outdoors in case there are fungus gnat eggs or larvae in it), and replace the medium with straight zeolite. So as not to disturb this protective mulch, I use the bottom-watering method.
I'm also making sure that my kitchen fruit-and-veggie peelings go quickly from under the sink to the outdoor compost heap.
While not wishing to appear smug, I can now say that my fungus gnat sightings over the past few weeks have dropped from "abundant" to "extremely few."
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