Please don't flame me for pointing this out, but the water will never be as sparkling as you want it to be, this is simply because the pond is planted and the fish will always stir things up, rooting around in the plants. Trust me on this you've got excellent water clarity for a planted pond, if everything reads fine then don't worry about it.
Forgive me for going a a bit peter waddy here but a koi pond doesn't have plants, a pond with plants is a pond with koi in it, if that makes sense.
If you want that extra sparkle the only thing you could add is filter floss which(apart from blanket weed) is the best and worse mechanical filter in the world. It takes everything out but at the same time can clog in no time resulting in a flood and empty pond. I've thought about it in the past and you could setup a baki shower type filter with and overflow with just floss in it. As you have no real issues I would just leave it as it is.
