It is going to happen sooner or later. Koi are living animals and all living things get sick occasionally. It is part of life.
When it comes to Koi you need to consider firstly why your Koi is sick. Is it a once off, or is it because there is something wrong elsewhere in your system?
The answer to this is easy. If it is just one fish that is ill, the chances are it is a natural occurrence. If there is more than one sick fish, the chances are that you are at fault because there is something wrong. If this is the case you cannot fail ro react fast enough. Speed is your greatest weapon. Use it.
In the event you have one isolated sick Koi in your pond the chances are very good it is not contagious.These include maladies that hit individual fish - genetic time bombs like heart attacks, strokes, degenerative diseases and the like. They are normally as fatal to Koi as they would be to us, and in most cases the sad truth is that there is little you can do about it.
It may be heart breaking but this is part of the hobby. If it happens to your most expensive fish it can also hurt your pocket considerably but this is also part of the hobby. An expensive fish doesn't mean it comes imbued with a greater health guarantee. Quite often the reverse may in fact be true.
Koi medicine is fairly primitive and expensive. Certain surgical procedures can be carried out, but as you might imagine, there are few Koi surgeons or vets out there suitably qualified to carry out these procedures and the likelihood of recovery is often small.
In the event that a Koi gets sick it is important to understand the following:
We believe many Koi are lost due to being killed by kindness more than by bad intentions.
Do not however assume that if a Koi is sick that the way to go is to leave it to its own devices. Quite often by the time a problem is picked up it may well be that the only chance a Koi has is for extreme treatments involving some handling. It happens to even experienced hobbyists that sometimes a Koi is overlooked. Don't beat yourself up about it - Koi are quite tough if kept in good warm water and the chances are good you will be able to effect a full recovery.
It does mean that a hospital tank (which can double up as a quarantine tank for new arrivals) is a really useful thing to have on hand under these circumstances. They are easy to set up and it enables you to remove the potential problem and the Koi patient from your Koi pond immediately. Such porta ponds are worth their weight in gold.