vegi filter
 

im thinking of growing watercress as a vegi filter. I have a header pool on
my main pond which is 10'x3' and 14" deep. I am thinking of growing
watercress in seed trays floating on the surface. What would you use as a
planting medium? I was thinking of mixing 2 parts gravel to 1 part clay cat
litter. Is this a good idea? My main pond holds 3500 gallons and is home to
11 koi and 4 goldfish.
thanks
Leigh