I have six small cherry stones in my both my tanks, and four piss clams I got from the acme (grocery store by me). A lot of other people say no to them and they may have valid reasons. I have had them for a long time with out a problem. When the area they are pointed in is to clean for them, they turn them selves around or move a little, some times burying their "mouth" in the sand. Most of the time they just lay with their "mouth" even with or right on top of the sand. Pretty cool looking, I think . When they open their "mouth" a tub comes out or it looks like ribbon candy going across their entire "mouth"