Quote:
Originally Posted by
MikeM 
Several decades ago the Japanese Inland Fisheries research institute studied what it was in hard water that made Sumi heavier compared to soft water. The key element identified was silicon. This may explain why mud ponds in sandy areas have been identified as good for Showa, while clay ponds elsewhere have seen to produce better Kohaku. Typically hard water has higher levels of silicon. In theory there could be low levels in hard water since "hardness" is measured without regard to silicon levels. Since silica is fairly ubiquitous in mineral composition, it would seem to me that this would be about the last thing to worry over. But, it is theoretically possible.
Some wild speculating to energize thinking: I would speculate that if a koikeeper had consistently poor Sumi development, but a neighbor had good Sumi development using the same tapwater, that there was a difference in husbandry and it might relate to use of clay. Clay mud ponds are often correlated with weak Sumi development compared to ponds in sandy soils. The Sumi comes up more dramatically when the fish raised in a clay pond are placed in a concrete holding tank. The water in these tanks is often drawn from the same source as the water filling the mud pond, and there is a continual flow which limits any change in hardness. But, there is no clay. The capacity of clay to adsorb mineral content from water could involve silicates being removed from circulation. ...This is pure speculation on my part. I've not seen any studies on the subject.