Hmm, how do they work?? I mean that the bottom is most important element in aquarium for good bacteria
I don't agree. While it is true that a portion of the biological colonies do inhabit the substrate, it is not the only element for colonizing bacteria beds. In my opiniopn a sponge filter can harbor more bacteria than a gravel bed. Also the bacteria will only colonize the first 1" or so of the substrate so anything deeper than this is basically useless for this purpose.
Now on to bare bottom tanks:
I believe Discus breeders have something in as much as their tanks are usually ALL bare bottom.
I myself keep my fry in bare bottom tanks...they grow like weeds and are heavily fed, thus producing much waste. In these tanks are simple sponge or box filters.
Sorry I just don't believe that the substrate is THE most important element for colonization of beneficial bacteria.