JHudson wrote in message
news:xrWUb.320$964.284@edtnps84...
Hi,
Just wondering what plants don't need a rich substrate, but take
everything
from the water.

Hornwort (Ceratophyllum submersum) and riccia grass (Riccia flutans) - if
you're looking for floating plants to suck up macro-nutrients (?) under
moderate lighting, either would be terrific. Water sprite (Ceratopterus
thalictroides), although usually rooted, also makes a fine floating plant.

Most true aquatic 'stem' plants, although they will develope complete root
systems, take nutrients readily from the water column. There are hundreds
available commercially.