[Ross]

Let me jump in with some additional observations from my orchidarium (retrofit 30 gal aquarium)

You don't need to provide outside air (air from the room). You must keep air moving constantly, though if there is no outside air coming in or out. Mine runs 24/7.

I occasionally get white mold started on a couple of the wet mounts, but Physan 20 stops that completely.

High amounts of light will get green algae growing pretty quickly and some pots may turn black on top from the algae.

Live green moss will start to make itself known on pots and/or mounts in a couple months and may grow completely over the mount. While pretty, it doesn't do the orchid any good, so I remove it.

As others have said, water will build up in bottom from misting, etc and grow additional agae. Need to remove it (as much as possible) with turkey baster or equivilant.

A high humidity terrarium (with misters, etc.) will probably not be good for any potted orchids, except perhaps Bulbos or Cischweinfias. It will be best for small mounted species such as Haraella or Gastrochilus.