"Old fish think" indeed. Compared to to the average funfair goldfish koi are far less resiliant.
Always have been always will.
Enough koi have been dumped into ponds and lakes over the decades by owners who have given up the hobby. Or koi in pools that have been in an areas of flooding which have escaped.
So where are they?
Why do you think koi owners have sophisticated filter systems, when the average golldfish can survive in a mucky pond?
This was the first post I made under the title "Koi's Law" a thread I started on a dedicated Koi message board, six years ago.
Not as an attempt to dissuade anyone from getting into this very addictive hobby, I thought I'd start a thread giving examples of the things with which we have to contend, where other contributors can add examples of their own experiences.
Here's a couple to start;
If you're building a pool or making any sort of alterations to one, as soon as you are part-way through it, it'll start raining.
If any koi decides to go "fins up" it's likely to be your favourite.
Others have added their experiences and the thread has had over 17,000 hits.
It wouldn't happen with goldfish.