Orchids (the domestic ones) in aquaria

The killiefish list has a discussion going on about the use
of orchids with the roots growing into aquariums. One chap
mentioned he has done this with "Phals" for some years.
Sounds fascinating! Any suggestions for orchid types which
might be compatible? There are many, many orchid species
and some prefer relatively cool temperatures. Under the
MH light might not be too good.