In the aquarium? I tried to get terracotta ones but they were completely out of the any that could fit in my tank. They had a pack of plastic terracotta coloured ones but unsure if these are safe?
I use some big plastic ones in my tank, which i cut in half and place slate on top of them, firstly to weight them down, secondly becuase they are easier for me to cut and hide under slate
never thought they would be a problem, ive used them now round two years, alls good