Does anyone know what "they" might be talking about in this post from freecycle.com? Besides growing to eatin' size, our goldfish seem to be doing quite well using our native "grey" stone.
"I am looking for rocks, the kind you use for a pond. Sand stone or anything else. They said not to use the grey rocks, not sure what they are called but you see them at the dam in Normandy. They do something to the water and it hurts the fish."
Freecycle by the way is a national group, but has locally administered chapters that trade things for free. You can not sell anything, or put in promotional statements, but you offer and can request items for giving away. It seems to be watched closely and is also helping many folks, so you might want to check it out.