Most ponds are now bottom drain designs which neatly circumvents your issue, As i am new to this hobby not sure you should really listen to me

, I would guess the issue would be how much speed is involved in it swimming under the pipe and how stupid your koi are. I would think that most fish are adept at avoiding submerged objects or there would be a distinct shortage of fish in the sea/river. The only issue then being the speed of impact with pipe, before I got my pond built around bottom drain design I too had a pipe trailing out of pond, fretted every time they swam under it, I have to say after 6 months none were any the worse for wear.
Hello, My names Brek, and I'm a Koiaholic.