Pond filter

Some time back I remember reading of an interesting filter application 
for ponds.  One component of this filter consisted of a tray(s) of 
aquatic plants such as water hyacinth or water lettuce whose roots acted 
as a biological filter media. If someone, with a mind less cluttered than 
mine, could please send details of the construction of the filter.  I 
have a friend interested in adding something like this to his pond setup. 

Stan Perkins

sperkins at admin4_hsc.uth.tmc.edu
phone (713)792-5800
fax  (713)794-4177