Any amount of dense rooted plants will work well in a VF. I love Hyacinth,
and Ingrid has had fantastic succes with celery. Water cress in a moving
body of water will grow like wildfire, but does not have as dense a root
system as the WH or the celery.

I will use hyacinth too... celery doesn't seem to grow for me. We have
some sort of nasty weeds in our livestock pond, so might get a few of
those and put in one corner of the trough.

I think your idea is great, except for one thing. Eventually that rock will
get clogged up. How will you clean it?

Everything is going to be enclosed in mesh bags, light enough for me
to lift out. (Hubby actually LOL)