A catalyst accelerates a chemical reaction. I think in this case, it is speeding along the sumi development. The sand itself is not affected in this process. But why would and how would silicon get into a koi, and what does it exactly do the speed up sumi development.
You mentioned something about melanin clogging the places where sumi developes. And this happens better and more affectively in nitrate free zones, or was it nitrite? well could the silicon be doing the same thing?