You CAN offer fish on occasion as a treat (crawfish are also well received - you can freeze the tails but not the whole animal. If feeding live, be sure it has been taken and killed within an hour so it does not die in the saltwater and pollute) as long as they were not fish kept in an aquarium (where copper is almost guaranteed as a treatment). ANY food from your fish market is fair game to try but limit the fish to treats as they don't contain enough of the needed fats for a regular diet. Typically, they will eat the soft parts of the fish and throw out the rest.