Hi, quick question. I added some of these smooth river rocks to my tank and, maybe I'm being paranoid, but I know a larger ray could toss a larger rock and maybe crack the tank. (Tank is acrylic.) The rays seem to like them and I can hear them playing with the rocks at times. What do you think? I read some previous threads on this and though I would see what others have experienced. Thanks in advance.[video=youtube;t12EakjOK08]http://www.youtube.com/watch?v=t12EakjOK08&list=UUVUBHxQcUl1Z-I3_yHZJz7A&index=1&feature=plcp[/video]