Simply Koi and More Koi

my pond is pumped unfortunately .my fish go under and over the pipe work from the pumps. is there any way to weight them down ,as i saw one koi get stuck for a moment going under 1st post in the room any help much appreciated. thank you

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.

Could you not make a pipe from solvent weld that tucks neatly into the corner. then connect this to the pump either with a short length of hose or screw type fitting.

Yep, solvent weld is the answer.
You either kink it to run on the bottom, which can collect debris, or raise it so fish can swim under.