Sounds like you may not have placed enough large rocks over the kitty littler and the koi are digging. The good news is it is 100% safe and healthy for the koi. I also believe you may have had some spawning taking place and that would have been perfect scenerio for them do there thing. Do not take the fish out and clean the pond, that is the worse thing you can do. Filter your water through fine matala material, batting material, or something similar. If you can vacumn the bottom a little at a time that too will help but don't try removing it all at once. leave all the floating plants in as long as possible (Until the koi eat them) you have some large koi and crowded for the size pond. My biggest suggestion would be consider building a bog so your plants will survive and still help clear the water. then start finding vegtables and fruit to feed the hungry guys. NO SEEDS, pretty much any other fruit or veggie can be fed. Watermelon, lettuce, greens, shelled peas, cucumber, squash. the list is endless. Some will be liked some will not. What they like in the spring may be different in the summer. Mine also like duckweed, i raise it in a tub and give them a hand full every once in a while. Keeping plants in a koi pond is possible, But you will have to make some adjustment to keep them out of the pots, either a cage or rough rocks on top of the planting media is a minimal to try with large koi.