We have a lot of torrential downpours and after a lot of rain my fish
don't eat for about a day or two...they just hover around not moving
much and when they do they try to jump out or are scratching at the
bottom of the pond like something is agitating them.

After about a day of "fasting" with "itching" all goes back to normal
like nothing ever happened. I have never lost a fish because of too much
rain....but it does affect them for sure.

Btw Isn't rain water soft?
As koi prefer hard water....