I have/had a couple of Koi ponds. One of them, I had 2 small firns, one on each side of the waterfall. That grew into monsters.
All year long, I had to climb in and pick off the ferns, that trailed along and through the waterfall rocks. Because the grew so rapidly and krept everywhere, that they would block the waterflow. I tried my best to keep the large plants as one large clump. Not letting any trailers over run my waterfall.
In my new pond, I left the ferns out this time
Too much work.
Good luck with yours though!
Pics of these ponds and the firns are in my photobucket link below. You'll see on the left side,
My Koi Pond