Battered fish recipes call for deep frying in 350 to 375 F oil, and 360 F is recommended if no temperature is specified. Oil thermometers measure exact temperature, and dropping a pinch of flour or bread cubes into the oil generally indicates if the oil is hot enough to fry.
Oil can be heated in a deep pot on the stove, in a Dutch oven pan placed in the oven, or in a deep fryer appliance. Frying fish in small batches about the surface area of the container prevents the oil's temperature from dropping when food is added. Some oils produce an unpleasant smoke when heated to deep frying temperatures. Battered fish needs to be fried in oil with a high smoke point, such as safflower, grapeseed, peanut or canola oil.