Thanks Jim,
the point is, I have barely 15 cm of vertical space, so not really much for anything which isn't "mini"...
The other point is, I wanted to keep the environment as close as possible to the real biotope, a low-end brackish river bank (i.e. no plants at all: the compromise with Java fern and Ceratophyllum you've seen is just for help fighting algae). Although, I know, tillandsiae and orchids surely do not grow that close to the water line....
Anyway, are you suggesting I keep the roots in water, and the leafs out? I believe at SG of 1.002 these sturdy plants should still be OK...