Actually I've been pondering on that one. I've forgotten more botany than I remember
but since various plantlife has different nutrient requirements I was thinking a bog/veggie prefilter stocked with the right plants could possibley consume some of the excess CaCO3 prior to hitting the Koi pond thereby lowering Kh to a more beni friendly level. Just a random thought. Of course if it would mean sacrificing the "mt.fuji, thai mountain waterfall oriental garden
" ... it would be a very poor trade off.