It is so very common that a person progressing from watergardener to koikeeper tries to compromise along the way by trying to find a way to keep aquatic plants with koi. Marketing to this desire, manufacturers of pond filters have come up with filter models that include a "plant filter", and magazine articles are written about once per year in the mass audience popular mags touting the benefits of vegetable filters, usually in the form of bog gardens through which koi pond water flows. In general, I think all of these contraptions and suggestions are a lot of silliness designed to sell "stuff", and not appropriate for koikeeping.
In theory, a plant filter would operate to soak up nitrogen from the koi pond, and other contaminants, resulting in improved water quality. However, I have never seen the theory implemented in a fashion appropriate for koikeeping. A successful plant filter would not have very healthy plants. That is because there would be so many plants that their nitrogen needs would be barely met. No plant would be grown in soil. Only inert media sufficient to stabilize the plant would be used. There would be tremendous work in maintaining the plant filter, because all dying leaves and roots would have to be immediately removed. Once a part of a plant begins to die, the release of the stored nitrogen into the system is very quick. Daily clean-up of the plants would be essential. Further, even if located after mechanical filtration, the roots and leaves will gather mulm, becoming the vegetative equivalent of a filter mat. Like any filter mat, they would need to be cleaned regularly. The pots would need to be emptied, the media cleaned of debris and the plants re-potted on a regular schedule. ...So much work to get plants that look poorly!
Of course, the idea of a vegetable filter is not sold on the basis of poor growing plants, but as a way to have beautiful aquatic plants and koi. Having one's cake and eating it, too.
So, what's the harm? In most situations, the veggie filter becomes a haven for build up of organic debris, rotting leaves, hydrogen sulfide production in the submerged soils, and all sorts of vermin that wait for the opportunity to parasitize the koi. A few folks may take the effort to minimize the risks, but in the main the chore becomes too much effort and the rotting mess lies just below the beautiful, fragrant waterlily blooms.
I wish the folks marketing some of these contraptions understood the needs of koi. But, they do not (or choose to ignore what they know), so it is up to the would-be koikeeper to understand that koi are not for water gardens, and half-way measures are only that.
P.S. I enjoy my waterlillies. They are in their own little pond where they can be potted in proper soil, fertilized when I feel like it and get all the sun they need to thrive.
