There are a lucky few who have never had to deal with koi illness and death. Koi generally stay healthy when water quality is maintained, feeding is appropriate and the koi keeper regularly observes the koi to spot problems right away.
So what can you do when you have koi exhibiting symptoms of illness or maybe you're finding dead koi in the pond? You can do some research online like the Koiphen health thread, you can ask for advice on the KKONA Facebook page or you can contact UMKC members for advice.
We have two certified Koi Health Advisors:
Koi can also be taken to the offices of Dr. Amy Kizer. Routine veterinarian rates will apply. Lexington Pet Clinic 651-452-5450
Some important things to know when you ask for advice:
(1) What are your water parameters?
(2) When did your koi start getting sick?
(3) Has anything changed before your koi started getting sick?
(4) Did you introduce any new koi or plants before your koi started getting sick?
SOS - My Koi are sick!!!
Presentation - Ron Sterbenz, UMKC Meeting 3.4.2017
Ron discussed maintenance/fixing pond equipment. Slide from his presentation can be seen in this PDF.
Websites of Interest
Koi Keepers of North America
Top Ten Koi & Pond Fish Keeping Mistakes https://youtu.be/UVjbMGgBCOQ
The Pond Co, Bloomington , MN
Koi Acres, Scandia, MN
Gertens, Inver Grove Heights, MN for EPDM and basic aquascape elements
Bachmans, various locations, MN
http://waterlandscapesupply.com - wholesale