I don't grow them in tanks but I've been reading about them... :check:

The tank provides humidity and warmth for the orchids inside. 10 gal. tanks would work. First, make sure it's clean, then fill the base with 1 in. of clay pellets (or some other medium), which will increase humidity. Then you simply place plants on top of the pellets (in their pots, I guess). Keep the clay pellets moist but make sure orchids are not standing in water. The tank should not be flooded and waterlogged, so when watering orchids you can take them out and let them drain before returning them back.

Plants good for this mini-greenhouse are miniature masdievallias and other small, slow-growing orchids (orchid experts, help me out here! ). You can also add small ferns/tillandsias to make it look nicer. A tank is a good idea! It's like a little greenhouse housing little orchids and making them feel nice and comfortable.

Note: I got the idea from a book but did not copy the words though!

Hope this helps!
I'm sure there are plenty of other ideas around so others can chime in too!