Japanese Takashi Amano has inspired many aquarium owners to keep the 1-5 cm tall cosmopolitic liverwort, Riccia fluitans, submerged. It can be kept down by tying it to a stone with a piece of fishing line, but new shoots always grow towards the surface, so it may be necessary to prune it with scissors.
 
Riccia fluitans thrives best with added CO2 and in good growing conditions small oxygen bubbles form on the leaf tips. As a traditional floating plant it offers good protection for young fish.