Is flashing when they swim on their sides in an attempt to "scratch" themselves on something? If so, my dats do that on occasion. Is their salt in the water or do you feed a lot of salt water shrimp? If so, this could be the case as that is when my dats tend to "scratch" more. Also, is their any driftwood in the tank? If their isn't, the dats could see the rays as the only to "scratch" on.