With all of the questionable keepers out there that will buy the cheapest stuff they can find, I think you'd see a LOT more issues with this if it WAS what was killing off snakes in batches. I personally still think it's got a lot to do with fish simply being contaminated with stuff as they are grown... seeing as our planet's shot to hell, I don't really buy into the whole "organic" movement as there realistically are no places untouched by human scum anymore. I've moved away from feeding fish filets because a whole prey diet is certainly a lot more ideal than filets with artificial supplements and it's not cost prohibitive anyways.