MOST of the fish that is prepackaged has some kind of sodium solution added to it either for freshness or the look of it i dont know how long ago they knew about it here at this forum. most people don't use wild caught fish for the reason of the parasites, but, i don't know about the mercury(i think that is the only heavy metal, in river fish at least) levels if they would hurt them. most people use bait fish like minnows or guppies or they get the fish fillets from the butcher section of the grocery. i'm sure you know most of this but i dont know if the OP knows it but after reading this he will know lol