They send down massive roots whilst floating on the surface, so can be rooted in a couple of feet of water whilst still floating. I've not seen them used in a aquarium before but they are designed to float, so if your meaning rooting them with the crown touching the substrate then I think it would be difficult unless you lowered the water level whilst they did so.
They do get big though at around 18 inches in diameter which rules them out of most tanks.
Thanks mort ive put it at substrate level not covering the crown lets see what happens ive got loads in my pond this year so ive just used one of the babies fingers crossed ive got huge tank so no issues with size
When I had them in my pond they rooted in the mud and seemed to sink lower in the water during the winter then float at the surface in summer. How this will affect aquarium use I don't know. As Mort has said they can get very big. When they reproduce by sending off plantlets you could use them 'till they out grow the space.
This is a natural method to stop them freezing in the winter. They are very common in the dykes around one of the broads and cab get very congested, the ones that don't sink see the leaves sticking out of the water die off.