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The Japanese Inland Fisheries guys did studies 20 or so years ago identifying silica as the mineral needed for sumi development. Silica is omnipresent, but higher in sandy areas. Going back to the early days of koi breeding, the breeders observed that the best sumi came from certain ponds, which often were in areas of higher sand content to the soil. Silicon levels in water tend to rise and fall with general hardness. It does not have to be correlated, but generally is.
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