I have always thought along the same line that Colin is pointing out.
My plan is to periodically (maybe 3-4x a year) treat my rays with anti bacterial and anit-parasitic meds even with out signs of a symptom. Of course I will use meds such as prazi pro that will be more then ray safe.
If I do this and freeze my catch (craws, minnows, shad, and left overs from fish I catch for my self!) I should be fine. However, what about pollutants such as mercury or any other man made pollutants?
Im really hoping to be able to safely feed my rays this because I figure a varied diet consisting of WHOLE foods that they would naturally eat would be best for my rays.