EH, use your wife's old pantyhose. They are great! I bought a box of knee highs to use, (cheap is good) and they stretch and you can put a LOT of kitty litter in them. I have a bag of it in my stream, anywhere that the water flows over it will help disperse it through the water.
Just an FYI for those of you who have or want to have koi, and worry about them tearing things up, so far my koi have done zero damage to any of my plants. I have no idea why they are well behaved, I would like to say that I taught them well, or spanked them when needed when they were young, but neither is true. haha
Also, a point to you, EH, having your pump on the bottom of your pond, I sure hope you have no lines from that pump that are on the outside of your pond liner, just in case a break ever happens in your line. Having the pump on the bottom (if it is literally on the bottom) will drain your pond in short order. I have my "bottom" pump on a milk crate, and that saved my fish a couple of weeks ago when my line pulled apart. Otherwise, I would have come home to an empty pond, and dead or dying fish. The other factor with a pump on the bottom is that it IS going to pull all the stuff off the bottom, and is more likely to get gunk clogged in the lines. I almost never have to clean my pump, it has no prefilter on it at all, the filter is in the Skippy before going back to the waterfall. Just a thought! I always thought pump on the bottom was best, keep that stuff circulating and out of the pond, but have since learned that the "gunk" on the bottom is also harboring beneficial bacteria. That's why this year I am not going to net the bottom until mid summer, or after all plants are growing really well, and the water has been warm enough for good bacteria to have a really good start. I think that was part of my problem last spring with murky brown water and algae. Also something I'm going to pay attention to this year. Sometimes cleaning too much is not good either.