Hi everyone. I have a 5K+ pond and I just have to give a big thumbs up to Watercress and all that it has done for my pond.
I have a bottom drain to settling pond. A 300 gallon skippy type filter, a 57 watt UV and two pumps that turn the pond over about every hour give or take. My source water is high in phosphates and I change about 10 percent every month. In the old days I had a pressure filter, the same skimmer with UV, settling pond, DIY small degassing chamber, twice weekly water changes and GREEN WATER! lol
Since I added floating planters with roughly 50 square feet of WC in the main pond, and WC in the filter and settling pond, and reduced water changes my water has been crystal clear with NEGLIBLE STING ALGAE since early spring. It is amazing and I highly recommend it. I only have 3 large Koi and two shubby mixes but I feed them at least from 4 to 8 ounces of high quality home made food from April to October.
I have attached a pic so that you can see that it is an attractive plant. It is in the right hand corner of the pic in a floating planter. The other floating planters also have some. Jenny (Koijoyii) uses it and also has it growing in the soil. It is an amazing pond plant and I wanted to share my experience with it.:)