Fish are affected by subsonics (below the range of human hearing)not ultrasonics. Subsonics create long waves which can damage tissue all the way through a fish.
If you have ever been in a room with a subwoofer blasting you can FEEL the vibrations in your body. With high powered subwoofers you may even feel uncomfortable from the pressure on your chest.
Water transmits these subsonic waves and carries them thousands of miles.
Ultrasonics can only be felt if you are actually touching the vibrating element.
Now if you had a whale or dolphin in your Koi pond the ultrasonics would bother them.