I fry fillets whole. Drop them into a 325 oil. When they float, they are done. One advantage of a bigger pot is that you can drop in more fish without cooling to oil too much. You should allow as much water to drain from them to keep the oil from splattering when the water boils. When I was a kid the firemen put on an annual fish fry to raise money. They had a huge cauldron (witch's style) with a water heater burner underneath. One guy would drop a fillet in on one side and it would go down one side and come up on the other side ready to eat.