Total hardness for keeping Koi
You are stating an "ideal value" of 6 - 16 °dGH for the total hardness in ponds. Is the upper value not too high, perhaps?
no, not at all. The common pond fish (Koi, goldfish, orfes, sturgeons etc.) can also be kept in hard water without any problems. A connection between hardness and coloration was detected in Koi: Red colors become particularly beautiful in soft water, black colors in hard water. Even in water with more than 30 °dGH - the source named in the following states 30 - 45 (!) °dGH, and England as an example for it - keeping Koi is "very successfuly"possible.
Source: Bernd Teichfischer, "Koi in den schönsten Wassergärten", S. 136 (Dähne Verlag 1996; in German language).
Other sources state 5 - 10 °dGH. Where does this difference you state come from?
The reasons are: Decades of practical experience, continuous contact with private pond/aquarium keepers, retailers, breeders, importers, widespread literature etc...
Dr. Bodo Schnell