Ray G decided to add:
I have a very small pond (mainly intended as a bird bath) and need
something to to keep it clean and tidy.
Is there a small water plant that would do the job?
TIA
Ray G
Try European Frogbit, or
Hydrocharis morsus-ranae.
Tiny lily like leaves with dainty white flowers June-August. These pretty
floaters hibernate in winter on the bottom of the pond and raise again in
May.
http://plants.ifas.ufl.edu/hydmor.html