Well, I hadn't heard of this either but I figured you all would have
seen something related if the use of these small stones as grinding
tools was a normal behavior with carp. I do see that sometimes folks
have reported a stone caught in the throat of a Koi. So we must assume
that some of them are either stupid enough to try to swallow a stone
that they could never pass or smart enough to use the stones like teeth
and the "caught in the throat" incidents are simply accidents during a
normal event. Since you all are mostly working with ponds, and the
smaller specimens in aquariums would not have a large enough mouth to
use stones in this manner, such an observaton might be rare even if the
behavior is common. Have stones been found in the stomach of this
species. Perhaps thought to have been swallowed as ballast?