One pollutant area that is often overlooked by pond keepers has to do with the plants in the pond or those that are part of the landscaping around (and over) the pond. The seeds of most plants can swell and plug up the digestive tract of the Koi. A partial list of the plants or parts of plants that have been reported as having some toxicity to Koi (and pets and people) for various reasons include:
Amaryllis - bulbs
Baneberry - berries, roots
Bird of Paradise - seeds
Black Locust Bark - sprouts, foliage
Boxwood - leaves, stems
Buttercup - sap, leaves
Calla Lily - leaves
Cherry - bark, twigs, leaves, pits
Coral Plant - seeds
Daffodil - bulbs
Datura - berries
Death Camas - all parts
Eggplant - all but fruit
Elephants Ear - leaves, stem
English Ivy - berries
Foxglove - leaves, seeds
Hemlock - all parts
Holly - berries
Hyacinth - bulbs
Indian Turnip - all parts
Iris - bulbs
Jasamine - berries
Java Bean - uncooked bean
Lantana - immature berries
Laurel - all parts
Locoweed - all parts
Marijuana - all parts
Mayapple - all parts
Mistletoe - berries
Mock Orange - fruit
Morning Glory - all parts
Narcissus - bulbs
Oak - acorns, foliage
Pine - sap
Poinsettia - leaves, flowers
Potato - eyes, new roots
Privet - berries, leaves
Prunus varieties - seeds, some
Redwood - sap (from decks also)
Rhubarb - leaves
Ranunculus - all parts parts
Snapdragon - all parts
Snowdrop - all parts
Tiger Lily - all parts
Tomato - leaves
Tulip - bulbs
Have you identified all the plants in, over, and around your pond?