I remembers a few years back (well I suppose it was decades) when one of our club members stopped and picked up some water hyacinths growing in a canal during a visit to california. He dropped them in his pond and came down with the worse case of anchor worm one could ever imagine....all this remembrance to warn, that if you do decide to get them and keep them out of the main pond so that the koi don't tear them apart and you still have them absorbing ammonia...be sure that no matter where you get them; that they are PP dipped to destroy anything on them.
When I first was learning filtration I used to keep them in the top of my filters. beautiful when they'd flower but after time it got so that the knowledge of filtration got better and better and with each new season and batch they didn't turn that beautiful deep green but a paltry yellow. I was glad cause then I knew the filter was robbing them of their food....
during this time your waiting for your filters to cycle, don't be fearful your koi will starve. feed them lightly with a low protein food.