trailing feces

The 2 goldfish are suddenly stricken with loooong body-length feces.
The weather loach not at all, fine.
It's been about a week now. Long black trailing things; they show no
stress signs, swim normally, fins erect and so on, feed normally.
Changed the diet 2 days ago to peas; they love them, and the feces are
now looong, trailing and green.
Water check: all normal, nitrates and trites minimal. Hardness is
hard, but always has been. What they eat has always been the same:
flakes and weekly live food. I can't think of anything that's changed
in the environment. Strange.