Failure to thrive happens regardless of what kinds of food items offered... No matter what kind of keeper you are, you are going to notice if a couple of your snakes die overnight after eating some fish. And you are right, if I were you I wouldn't eat the fish either... In fact avoid any meat at a grocery store! Meats don't belong on a conveyor belt under a series of sprayers. Like I said, if one is really worried about it, I would just avoid feeding filets... Kind of like thiaminase! There are plenty of feeding options that are out there.