Koi died after water change

[MimsBird]

I never do water changes, but you can get away with doing that if you do it correctly.

The biggest point is that doing a complete drain and scrub is really bad. Especially the scrubbing or power washing.
That existing water is liquid gold. It contains everything the fish need for a healthy existence.
It's not just the beneficial bacteria and biofilm you are getting rid of, but there's also many microorganisms that are part of the ecosystem. Fish cannot survive in a sterile environment.

I agree that the size of the pond is a huge factor. A small pond reacts to changes quicker and more detrimental than a bigger pond.
If your pond is overstocked with fish, that's a big concern and makes things exponentially worse.

The same water has been in my pond for years. Nature adds to my pond when it rains. I can see if you live in an area where there isn't much rain and you suffer with severe evaporation. In that case, you may need to add water. If your home's water is chlorinated, you need to add the proper treatment.

Mercifully I only had five they be koi in there. But all in all they cost me 100 bucks at the time that I got them. I know it could’ve been thousands down the drain if I had bought larger Koi. Going forward I’m going to have all the bells and whistles and I’m currently about to open and try the API master pond water testing kit.