By the way.... 12 volt fans can be placed entirely inside the tank blowing lots of air about without any air exchange to the outside. Just a thought. That's what I am doing with mine in this picture:
There is no outside air exchange unless I open the doors.
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Ross
http://orchids-ross.blogspot.com/
I don't do most Dendros or large, "floofy orchids". For me it's minis (like Angraecoides), Masdies, Paphs, and others that crept in somewhere along the line. See my orchid list for complete collection (usually not current.)