I have about a dozen golfball to baseball sized smooth river rocks in the tank with my rays (with a nice amount of smooth sand as well) and my rays actually move the rocks to the front of the tank and constantly toss them against the acrylic when they get hungry (or maybe for entertainment too). They will do this if I am around or not. When they see me, they just pile up and beg against the front of the tank like dogs. Someday I will take some videos of this, it just blew me away how they move the rocks around and toss them against the side.