I have a couple 8-9" motoros in my 220g with pool filter sand substrate. The substrate is about 3/4-1" thick and I've noticed that they tend to circle the open space in the tank, creating a "track" around the outside edge. This behavior seems to cause the sand to pile up in the center of the tank. I just found this a bit curious and I was wondering if anyone else had observed this behavior from their rays.