Koi Diseases and Parasites
Ways Parasites & Diseases get into your pond
Everyone talks about quarantinging new fish that are introduced to your pond, but you might not realize that almost all koi carry diseases with them, sort of like how humans carry strep throat. And just like strep, when the pond conditions deteriorate, the koi get stressed, and some diseases can start to emerge and affect your fish.
Birds also transmit many things in and out of your pond. They stand in the water, dip their beaks, splash around etc. Parasites often stick to the birds, then when they go to the next pond, the parasites get dropped off. Some of the stuff is transmitted through their poop.
Predators that will eat your fish
Herons roam all over Phoenix and can clean out your pond in a day or two. They spot your fish from high in the air, land in your yard and then walk into the shallows of your pond. They stand motionless till the fish forget they are there, then snatch the one which comes close.
To defend against these, you can build your pond with straight drops to the walls atleast 2' deep, and 3' is better. Bird netting, covering the pond is the ultimate protection, but many people do not want to cover their ponds. I have seen some suggest motion activated water squiring things, fake alligators and other type decoys. There is even a floating fake koi, and the way it is supposed to work is the heron strikes the floating koi first, and the other fish are then alerted to the heron and go hide.
Herons are really smart, but there are 2 techniques I have heard of from people that were fending off herons: trip wires & aerial fishing string. Evidently, the herons do not like to step over trip wires, and so if you have them around the perimeter of your pond about a foot off the ground, that is supposed to stop them. The aerial fishing string is supposed to be suspended about 10 feet off the ground above and around the area they would aproach to land in your yard. The fishing string being so hard to see is supposed to really make them mad if they strike it, and it will scare them off.
Raccoons are also heavy predators, they wander into the shallows and wait for the fish to swim close, a similar tactic that the herons do. So the steep walls help prevent racoons too. And yes, we have raccoons in Phoenix.
Koi Disease Identification and Treatment
I am not a koi health expert. I have been to a couple of seminars, have read a few koi health books, and have treated numerous fish and cured them of common problems, mostly with "shotgun" aproaches using multiple medicines at the same time. The real experts scrape fish & take clipping samples, examine those under a microscope to detect parasites so they can do specific treatments. Also a professional can take blood and fecal samples to have analyzed by a lab to determine exact baterial problems.
I highly suggest you get some books about koi health. Some of the information in the books contradict each other, so what I do is have several of them. When I have a problem, I read the apropriate sections in each one to try and figure out the best course of action. The book I use the most is the Manual of Koi Health (see it in the list of books below).
The books also have pictures of koi that are affected by the various diseases, so if your koi starts to have a problem, you can look in the books to find a matching sick koi and treat accordingly.