Koi and Plants
When keeping koi, it is important to know that koi are primarily herbivorous in nature and thus plants are often not an option. However, if you have an area where koi are unable to enter, (i.e. sump filter, waterfall, floating baskets) plants can be safely be placed without risk of becoming food for the koi.
Even if plants are placed in a suspended basket grown aqua/hydroponically, koi can persistently attempt to knock your plants into the water and in rare occasions succeed. Sump filters are often the best place to grow vegetables or other plants as it is nutrient rich and far from the hungry mouths of your koi.
If you are willing to risk a large buffet for your koi, when using floating plants like Water Hyacinth (Eichhornia crassipes) and Water Lettuce (Pistia stratiotes), these plants can choke the water of oxygen if not access to air it available. Water Hyacinth can also rot and decay causing pollution in your pond in areas with long winter months. Plants like Anacharis (Egeria densa) will also spread rapidly and can look unsightly if every inch of the pond is filled with this aquatic weed.
Occasionally it is important to check the roots of your plants as koi can sometimes eat the roots but leave the leafy portions alone; rootless plants can potentially die causing sudden spikes in ammonia and nitrite.
Plants collected from the wild can potentially carry unwanted pests like crayfish, dragonfly nymphs, and eggs of native (or invasive) fish. Make sure to check your local laws regarding which species of plants you are allowed to own and whether you can collect them from the river or streams; never flush unwanted plants down the drain or release them into the wild. This is illegal anywhere where laws exist.