2 of my 3 ray tanks all have fine gravel in them and none of the rays seem to pick on the others. Lately my tank without fine gravel there are a couple leos that seem to just go around biting the tail and the disk of the other rays just for the fun of it, like when two kids are bored and just start picking at each other. So I introduced some fine gravel in the tank 3 days ago and the biting has stopped so far. The rays seem preoccupied just blowing in the sand. Not sure if this will last but makes you wonder how bored they are in bare bottom situations.