Is a deep cast iron skillet the best for frying fish?

Cast iron, if properly seasoned and cared for, is Excellent for cooking and baking (cornbread, pineApple upside down cake, etc.) because trace iron is conveyed to the food. You can use an iron pot for frying, but it is unlikely that the benefits will be transfered through the hot oil into the fish. You can use any heavy, deep pot for frying. Make sure you have at least as much pot above the oil as below to allow for the addition of food and to keep spatters to a minimum.